crystals Article
Experimental Study on the Oxidation and Diffusion Behavior of Inconel 625 and Tool Materials Erliang Liu 1,2, * , Ning Wang 2 , Jin Qi 2 , Zhichao Xu 2 , Xia Liu 2 and Huiping Zhang 2 1 2
*
College of Mechanical and Power Engineering, Harbin University of Science and Technology, Harbin 150080, China Laboratory of Advanced Cutting and Tools Technology, Harbin University of Science and Technology, Harbin 150080, China;
[email protected] (N.W.);
[email protected] (J.Q.);
[email protected] (Z.X.);
[email protected] (X.L.);
[email protected] (H.Z.) Correspondence:
[email protected]; Tel.: +86-139-4605-0530
Received: 19 October 2018; Accepted: 13 December 2018; Published: 15 December 2018
Abstract: Oxidation and diffusion simulation experiments were conducted to choose the most suitable material for cutting the Inconel 625 superalloy. Three tool materials, WC/Co, coated carbide, and ceramic were used as tool materials in the oxidation simulation experiment. The three tool materials were heated for 30 min in a high-temperature furnace, and the high-temperature oxidation products were examined with scanning electron microscopy and X-ray diffraction (XRD). Tools were heated for 90 min in a vacuum tube furnace. The element diffusion behaviors of Inconel 625 and the tool materials were analysed with energy-dispersive X-ray spectroscopy and XRD. Some of the WC and Co in the WC/Co and coated carbide tool materials was oxidized to WO3 , Co3 O4 , and CoWO4 , and the oxidation reaction became more intense as the temperature increased. For the ceramic tool, only TiC was oxidized to TiO2 , which indicates good oxidation resistance. In the diffusion couple experiments, the diffusion levels of the three tool materials increased with temperature, but the degree of influence differed. Diffusion of elements was hindered by the (Al, Ti) N coating of the coated carbide and effectively inhibited by the Al2 O3 in the ceramic tool. In terms of oxidation and diffusion, the most suitable tool material for cutting Inconel 625 was the ceramic, followed by the coated carbide and then WC/Co. Keywords: Inconel 625; WC/Co; coated carbide; ceramic; oxidation; diffusion
1. Introduction Inconel 625 is widely used in engine parts because of its good corrosion resistance and thermal fatigue properties [1]. Several characteristics, including high toughness and ductility, high melting point, excellent resistance to corrosion, thermal shocks, thermal fatigue, and erosion are primarily responsible for its wide domain of application [2]. However, its low thermal conductivity, high strength, and high chemical reactivity limit its machinability [3]. The selection of tool materials for cutting Inconel 625 involves even higher requirements. Tool materials used for cutting Inconel 625 have mainly been WC/Co, coated carbide, ceramics and PCBN (polycrystalline cubic boron nitride). PCBN has been less used because of its high price and because it is easy to produce the phenomenon of tipping when cutting superalloy. Such materials need to allow the tool to have processing accuracy while also improving the processing efficiency and tool life. Many scholars have researched and theorized on the optimization of tool materials and machining wear [4,5]. Deng et al. analysed the wear mechanism for cutting titanium alloy and stainless steel and optimized the most suitable tool for cutting Ti6 Al4 V and Cr12 Mn5 Ni4 Mo3 Al [6]. Lotfi et al. performed 3D finite element simulations to analyze the wear of coated carbide (Inconel 625) and confirmed that
Crystals 2018, 8, 471 ; doi:10.3390/cryst8120471
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the cutting temperature and cutting force are the two most important factors that limit tool wear [7]. Pan et al. examined the high-speed friction wear characteristics of ultrafine-grained carbide tools and determined the main wear mechanisms to be abrasive and adhesive wear with a small amount of diffusion wear [8]. Zhu et al. studied the tool wear characteristics for cutting the nickel-based superalloy Inconel 718 and proposed that diffusion and oxidation wear were the main wear patterns [9]. Martinez et al. performed an Inconel 718 cutting experiment with TiN- and TiAlN-coated carbide tools. They found that high-speed cutting of a superalloy produces a higher cutting pressure and temperature, while a high cutting temperature was the main factor that induces tool oxidation and diffusion wear [10]. Xavior et al. cut a nickel-based superalloy with carbide tools and found that the main factors that affect tool wear were thermal softening, bonding, diffusion, and groove and thermal cracking of the tool material under high temperature and stress [11]. Comprehensive research has shown that most materials used in low-speed cutting were mainly worn by adhesive wear and oxidation. During the high-speed cutting process, it was difficult for oxygen to enter the contact area of the tool tip. Wear was mainly in the form of diffusion wear under anoxic conditions [12]. However, research on the oxidation and diffusion of tool materials has been limited to phenomena without deep mechanisms, and there have been no systematic studies on the oxidation and diffusion behaviour of Inconel 625 and tool materials. In this study, Gibbs free energy was used to predict the possible oxidation reactions and products of different tool materials at high temperatures, and oxidation simulation experiments were performed with a diffusion model of each tool material and Inconel 625. Inconel 625 was constructed using Fick’s law, and diffusion couple experiments were conducted. The samples were examined with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to analyze the oxidation behaviour of the tool materials and their diffusion behaviour with Inconel 625. This research provides evidence for the selection of tool materials for cutting Inconel 625. 2. Experimental Procedures WC–Co cemented carbide, WC–Co cemented carbide coated with (Al, Ti) N, and Al2 O3 –TiC ceramic were selected for the tool materials, and Inconel 625 was selected as the workpiece material. It has been observed that the cutting temperature when machining such superalloys can reach a maximum of about 1273 K [13]. Tables 1 and 2 present the specific components of the tool and workpiece materials, respectively. The information of different materials came from manufacturers: Inconel 625 from Shanghai Chongao Industry Co., Ltd (Shanghai, China); WC/Co from Xiamen Weisina Coating Technology Co., Ltd. (Xiamen, China); coated carbide from Mitsubishi Tools Co. (Shanghai, China); and ceramics from Sandvik Co. (Gimo, Sweden). The tools were cleaned with acetone, blown dry, and weighed on an electronic balance (JA5003, measurement accuracy: 0.0001g). Tool materials were heated for 30 min in a high-temperature heating furnace (pot resistance furnace, SG2-5-10) at 873 K, 973 K, 1073 K, 1173 K, and 1273 K before being cooled to room temperature. The specimens were then taken out and weighed with an electronic balance before the experimental data were recorded. The oxidation products and surface morphology of the specimens were observed with XRD (LabX xrd-6000, Acc.V: 20.00 KV, Current: 100 mA, step size: 0.02 deg, Cu Kα rays, scanning speed: 4 dg/min, SHIMADZU, Kyoto, Japan) and SEM (HITACHI SU3500, Spot: 40, Acc.V: 20.00 KV, Magn: 2000 X, HITACHI, Japan), respectively. Cylindrical specimens of the tool and workpiece materials with dimensions of ϕ = 8 mm × 10 mm were used in the diffusion couple experiments. The tool specimens were lapped, polished, and cleaned with acetone. Then, the workpiece and tool specimens were placed in contact with each other to form a diffusion couple, as shown in Figure 1. A clamping force of 10 MPa was maintained on the specimens to ensure a smooth contact surface. The function of pressure was to make the plastic deform near the contact interface to obtain close contact so that elements could diffuse. It has been observed that the cutting temperature when machining such superalloys can reach a maximum of about 1273 K. Hence, the clamped specimens were placed in a vacuum tube furnace and heated for 90 min at 873 K, 973 K,
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1073 K, 1173 and 1273 K.of EDS XRD were used to detect and compounds of contents andK,compounds theand specimens, respectively, andthe theelement elementcontents diffusion coefficients of the the specimens, respectively, and the element diffusion coefficients of the tool and workpiece materials tool and workpiece materials were calculated. were calculated. Table 1. Percentage composition of the three tool materials. Table 1. Percentage composition of the three tool materials.
Tool Material
Chemical Composition (wt%) Chemical Composition WC Co (wt%) WC/Co WC Co 92 8 WC/Co 92 8 Coated Carbide C Al W Ti N C Al W Ti N Coated Carbide (CNGG120408) 8.19 (CNGG120408) 42.8 6.79 8.19 10.9110.91 31.31 31.31 42.8 6.79 Ceramic C Al O Ti C Al O Ti Ceramic (CNGA120408) (CNGA120408) 12.08 12.08 39.62 39.62 31.57 31.57 16.73 16.73 Tool Material
Table2.2.Element Elementcomposition compositionofofInconel Inconel625. 625. Table Element Content% Element content% Min Min Max Max
NiNi
Cr Cr
Mo Mo
Nb Nb
Fe Fe
Al Al
5858 -
20 20 23
88 10
3.15 3.15 4.15
-5
-0.4
-
23
10
4.15
5
0.4
Ti Ti -0.4 0.4
CC -0.1 0.1
Mn Mn -0.5 0.5
Si Si -0.5 0.5
Cu P Cu P -0.5 0.015 0.5 0.015
S - 0.015
0.015
Tool material Inconel 625
(a)
(b)
Figure 1. 1. Clamping Clamping of of the the tool tool material material and and Inconel Inconel 625 625 specimen. specimen. Figure
3.3.Results Resultsand andDiscussion Discussion 3.1. 3.1.Oxidation OxidationBehaviour Behaviourofofthe theTool ToolMaterial Material 3.1.1. Theoretical Analysis of the Oxidation Reaction 3.1.1. Theoretical Analysis of the Oxidation Reaction Table 3 presents the chemical reaction of the tool matrix with oxygen at high temperatures during Table 3 presents the chemical reaction of the tool matrix with oxygen at high temperatures the superalloy cutting process. Some of the oxidation products were not stable at high temperatures. during the superalloy cutting process. Some of the oxidation products were not stable at high In Table 4, chemical reactions are presented that easily occurred with other substances. temperatures. In Table 4, chemical reactions are presented that easily occurred with other substances. Table 3. Possible oxidation reactions on the tool. Table 3. Possible oxidation reactions on the tool. Tool Components
Tool Components
WC, Co, TiC
WC, Co, TiC
Possible Oxidation Reactions
Possible Oxidation Reactions 2WC + 5O = 2WO + 2CO 2WC + 5O22= 2WO33 + 2CO22 2TiC + 3O2 = 2TiO + 2CO2 2TiC 2CO2 TiC++3O 2O22 ==2TiO TiO2 ++ 2CO 2 TiC + 2O 2 = TiO 2 + 2CO WC + 2O2 = WO2 + CO22 + 22O WC3Co + 2O = WO 2 +3 O CO 2 = Co 4 2 2Co + O = 2CoO 3Co + 2O22= Co3O4 2Co + O2 = 2CoO
Gibbs Free Energy (kJ)
Gibbs Free Energy (kJ) −2100.4 −2100.4 −1482.9 −1482.9 −1757.5 −1757.5 −855.7 −525.8 −855.7 − 361.2 −525.8 −361.2
Table 4. Possible reactions with the oxidation products.
Oxidation Products Co3O4, WO3, CoO
Possible Reactions Co3O4 + 0.5C + 3WO3 = 3CoWO4 + 0.5CO2
Gibbs Free Energy (kJ) −3073.2
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Table 4. Possible reactions with the oxidation products. Oxidation Products
Possible Reactions
Crystals 2018, 8, x FOR PEER REVIEW Co3 O4 + 0.5C + 3WO3 = 3CoWO4 + 0.5CO2
Co3 O4 , WO3 , CoO
Co3 O4 + C + 3WO3 = 3CoWO4 + CO WO33 == 3CoWO CoWO4 4 + CO Co3O4 +CoO C + +3WO CoO + WO3 = CoWO4
Gibbs Free Energy (kJ)
−3073.2 −2852.2 −853.6 −2852.2
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−853.6
According to the second law of thermodynamics, if any chemical reaction can proceed Accordingthetofree theenergy second lawreaction of thermodynamics, any chemical reaction proceed spontaneously, in this process must beifreduced. The specific basiscan for judging spontaneously, the free energy in this reaction process must be reduced. The specific basis for judging a spontaneous response is as follows: if ∆G < 0, then the reaction proceeds spontaneously; if ∆G = 0, a spontaneous response is as follows: if ∆G ＜ 0, then the reaction proceeds spontaneously; if ∆G = 0, then the reaction reaches equilibrium; and if ∆G > 0, then the reaction does not occur. thenThe the Gibbs reaction equilibrium; ∆G ＞ 0,then the reaction does not as occur. freereaches energies of the tool and and if workpiece reactions were calculated follows: The Gibbs free energies of the tool and workpiece reactions were calculated as follows: 0 0 ∆G∆𝐺 ∅0T T ==∆H ∆𝐻298 −−T∆ T∆∅
(1) (1)
0 is∆𝐻 the Gibbs free energy in the reaction, is the reaction heat T is the wherewhere ∆GT0 is∆𝐺 the is Gibbs free energy in the reaction, ∆H298 the reaction heat effect, T iseffect, the absolute 0 0 0 absolute temperature, and ∆∅ is the Gibbs free energy function. In Equation (1), ∆∅ and ∆𝐻 temperature, and ∆∅T . is the Gibbs free energy function. In Equation (1), ∆∅T and ∆H298 change change the temperature T. The free energy andheat reaction of the toolaccording materials with the with temperature T. The Gibbs freeGibbs energy and reaction effect heat of theeffect tool materials according a relevant[14] handbook [14]towere used obtain Gibbsatfree the relevant to a relevanttohandbook were used obtain theto Gibbs freethe energy the energy relevantattemperature. temperature. Tables 3the and 4 present the Gibbs theK.tool body at 1200 Tables 3 and 4 present Gibbs free energy of thefree tool energy body atof 1200 Comparison withK. theComparison Gibbs free with theofGibbs free energiesshowed of the tool showed TiC can more easily oxidize to TiO energies the tool materials that materials TiC can more easilythat oxidize to TiO than to TiO and that Co 2 2 than to TiO and that Co more easily oxidise to Co 3O4 than to CoO. Furthermore, a new chemical can more easily oxidise to can Co3 O than to CoO. Furthermore, a new chemical reaction occurred between 4 reaction occurred betweenCoWO the intermediate products 4 and COused 2. The analysis can be the intermediate products above CoWO analysis can be toabove predict the chemical 4 and CO2. The used to with predict the chemical reaction Asforms, WO3 has gas WO and3solid forms, reaction relevant compounds. Aswith WO3relevant has bothcompounds. gas and solid the both gaseous escapes to the gaseous WO 3 escapes to the air. In this study, the Gibbs free energy values of the solid WO 3 and the air. In this study, the Gibbs free energy values of the solid WO3 and intermediate products were intermediate products were calculated. calculated.
3.1.2. 3.1.2.Analysis AnalysisResults Resultsfor forthe theOxidation OxidationReaction ReactionExperiment Experiment Figure Figure2a–c 2a–cshows showsthe theXRD XRDpatterns patternsofofthe theWC/Co WC/Co tool tool at at 873 873 K, 973 K, and 1273 K, respectively. respectively. When Whenthe thetemperature temperaturewas was873 873K, K,there therewere wereno noobvious obviousoxidation oxidationproducts. products.When Whenthe thetemperature temperature rose to 973 K, WO and Co O were observed, and CoWO was produced by the combined rose to 973 K, WO reaction 3 3 and Co 3 3O44 were observed, and CoWO44 was produced by the combinedreaction between betweenWC WCand andCo. Co.However, However,when whenthe thetemperature temperaturerose roseto to1273 1273K, K,WC WCstill stillshowed showedaadiffraction diffraction peak peakamong amongthe theoxidation oxidationproducts, products,indicating indicatingthat thatititwas wasnot notcompletely completelyoxidized. oxidized.
(a)
(b)
(c)
Figure2.2. X-ray (XRD) patterns of WC/Co at different temperatures: (a) 873 (a) K, WC: Figure X-raydiffraction diffraction (XRD) patterns of WC/Co at different temperatures: 873 PDF#65K, WC: 4539; Co: PDF#65-9722; (b) 973 K,(b) WO 3: PDF#71-0305; CoWO4: PDF#72-0479; Co3O4: PDF#42-1467; PDF#65-4539; Co: PDF#65-9722; 973 K, WO3 : PDF#71-0305; CoWO4 : PDF#72-0479; Co3 O4 : 4: PDF#42-1467; CoWO4: PDF#72-0479; WO3: PDF#71-0305; WC: PDF#65-4539; and (c) 1273 and K, Co PDF#42-1467; WC: PDF#65-4539; (c)3O1273 K, Co3 O4 : PDF#42-1467; CoWO4 : PDF#72-0479; WO3 : WC: PDF#65-4539. PDF#71-0305; WC: PDF#65-4539.
Figure Figure3a–c 3a–cshows showsthe theXRD XRDpattern patterncharts chartsfor forthe thecoated coatedcarbide carbidetool toolmaterial materialatattemperatures temperatures of of973 973K, K,1073 1073K, K,and and1273 1273K, K,respectively. respectively.Owing Owingtotothe thecoating’s coating’sblocking blockingeffect, effect,the thecoated coatedcarbide carbide tool showed no obvious oxidation products when the temperature was between 873 K and 1073 tool showed no obvious oxidation products when the temperature was between 873 K and 1073 K. K. When the temperature rose to 1273 K, however, not only oxidation products of the tool (i.e. WO3, Co3O4, and CoWO4) but also oxidation products of the coating (i.e. Al2O3 and TiO2) were detected. This was mainly because the oxidation reaction between the coating and matrix composition of the tool occurs at high temperatures.
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When the temperature rose to 1273 K, however, not only oxidation products of the tool (i.e., WO3 , Co3 O4 , and CoWO4 ) but also oxidation products of the coating (i.e., Al2 O3 and TiO2 ) were detected. Crystals 2018, FOR PEERREVIEW REVIEW 5ofof This was mainly because the oxidation reaction between the coating and matrix composition5of the Crystals 2018, 8,8,x xFOR PEER 1313 tool occurs at high temperatures.
(a) (a)
(b) (b)
(c) (c)
Figure3. XRDpatterns patternsofof the coated carbide tool atdifferent different temperatures: (a)873 873WC: K,WC: WC:PDF#65PDF#65Figure the coated carbide tool temperatures: K, Figure 3.3.XRD XRD the coated carbide tool atatdifferent temperatures: (a)(a) 873 K, PDF#65-4539; 4539; Co: PDF#15-0806; (Al, Ti)N: PDF #16-0867; (b) 973 K, WC: PDF#65-4539; Co: PDF#15-0806; (Al, 4539; Co: PDF#15-0806; Ti)N: PDF #16-0867; 973 WC:PDF#65-4539; PDF#65-4539; Co: Co: PDF#15-0806; Co: PDF#15-0806; (Al,(Al, Ti)N: PDF #16-0867; (b)(b) 973 K,K,WC: PDF#15-0806; (Al, (Al, 3 : PDF#71-0131; CoWO 4 : PDF#72-0479; WC: PDF#25-1047; TiO Ti)N: PDF #16-0867; (c) 1273K, WO Ti)N: Ti)N: PDF PDF #16-0867; #16-0867; (c) 1273K, WO33: PDF#71-0131; PDF#71-0131; CoWO CoWO44: : PDF#72-0479; PDF#72-0479;WC: WC:PDF#25-1047; PDF#25-1047;TiO TiO22: 2:: 2 O 3 : PDF#16-0394; Co 3 O 4 : PDF#42-1467). PDF#23-1446; Al 3O 4: 4PDF#42-1467). PDF#23-1446; PDF#23-1446;Al Al22OO3:3PDF#16-0394; : PDF#16-0394;Co Co : PDF#42-1467). 3O
Figure4a–c 4a–cshows XRD diagrams the ceramic material temperatures 973, 11731173 and Figure 4a–c showsXRD XRDdiagrams diagrams the ceramic material at temperatures of 973, K, Figure ofofof the ceramic material atattemperatures ofof973, 1173 K,K,and 1273 K, respectively. When the temperature was 973 K, the tool showed no obvious oxidation and 1273 K, respectively. When the temperature was 973 K, the tool no obvious oxidation 1273 K, respectively. When the temperature was 973 K, the tool showed no obvious oxidation products.When Whenthe thetemperature temperaturerose roseto to1173 1173K, K,the theTiC TiCof ofthe thetool toolproduced producedthe theoxidation oxidationproduct product products. When the temperature rose to 1173 K, the TiC of the tool produced the oxidation product products. TiO 2 , but TiC was still present. When the temperature rose to 1273 K, only TiO 2 was observed, with TiO , but TiC was still present. When the temperature rose to 1273 K, only TiO was observed, with no TiO22, but TiC still present. When the temperature rose to 1273 K, only TiO 2 2 was observed, with no TiC. Thus, the TiC in the ceramic tool was completely oxidized to TiO 2 , which is also why the color TiC. Thus, thethe TiCTiC in the ceramic tool was completely oxidized to to TiO is is also why thethe color of no TiC. Thus, in the ceramic tool was completely oxidized TiO 2, which also why color 2 , which ofthe the ceramic material changed from cinereous white. The matrix 2O 3of ofthe theceramic ceramic tooldid did the ceramic material changed from cinereous to white. TheThe matrix Al2Al OAl of ceramic tooltool did not of ceramic material changed from cinereous totowhite. matrix 3 the 32O notchange. change. Therefore, theceramic ceramic toolshowed showed strong antioxidant properties. change. Therefore, the the ceramic tooltool showed strong antioxidant properties. not Therefore, strong antioxidant properties.
(a) (a)
(b) (b)
(c) (c)
Figure4. XRDpatterns patternsof ofthe theceramic ceramictool tooloxidation oxidationproducts productsat atdifferent differenttemperatures. temperatures.(a) (a)973 973K, K, Figure 4.4.XRD XRD patterns of the ceramic tool oxidation products at different temperatures. (a) 973 K, Figure Al O : PDF#70-3319; TiC: PDF#65-8803; (b) 1173 K, Al O : PDF#70-3319; TiC: PDF#65-8803; TiO 2 O 3 PDF#70-3319; TiC: PDF#65-8803; (b) 1173 K, Al 2 O 3 : PDF#70-3319; TiC: PDF#65-8803; TiO Al Al22O3:3 PDF#70-3319; TiC: PDF#65-8803; (b) 1173 K, Al22O33: PDF#70-3319; TiC: PDF#65-8803; TiO22: 2:: PDF#21-1236; (c) 1273 K, Al :: PDF#16-0394;TiO TiO : PDF#21-1236; PDF#21-1236. O TiO PDF#21-1236;(c) (c)1273 1273K, K,Al Al 22 O2O 3:33PDF#16-0394; 2:22PDF#21-1236; PDF#21-1236;
Figure shows the oxidation mass gain curves of different tool materials different temperatures. Figure555shows showsthe theoxidation oxidationmass massgain gaincurves curvesof ofdifferent differenttool toolmaterials materialsatat atdifferent differenttemperatures. temperatures. Figure When the temperature was lower than 1000 K, there was no obvious mass gain due to tool material When the temperature was lower than 1000 K, there was no obvious mass gain due to toolmaterial material When the temperature was lower than 1000 K, there was no obvious mass gain due to tool oxidation. Around 1000 K, the oxidation mass gain phenomenon appeared. As the temperature rose, oxidation.Around Around1000 1000K,K,the theoxidation oxidationmass massgain gainphenomenon phenomenonappeared. appeared.As Asthe thetemperature temperaturerose, rose, the oxidation. the the mass increased sharply owing to oxidation. The peak mass gain occurred when the temperature mass increased sharply owing to oxidation. The peak mass gain occurred when the temperature rose mass increased sharply owing to oxidation. The peak mass gain occurred when the temperature rose toto rose to about 1200 K. mainly This was mainly because of WO3 at sublimation at the atgenerated 1100 K. about 1200 Thiswas was mainly because WO 3 sublimation atthe thetool toolsurface surface 1100surface Thisgenerated about 1200 K.K.This because ofofWO 3 sublimation atattool 1100 K.K.This This generated extensive contact between the tool matrix and oxygen in the air, which sharply increased extensivecontact contactbetween betweenthe thetool toolmatrix matrixand andoxygen oxygenininthe theair, air,which whichsharply sharplyincreased increasedthe theoxidation oxidationofof extensive the oxidation of the tool material. As the temperatures rose, the oxidation mass gain began to decrease. the tool material. As the temperatures rose, the oxidation mass gain began to decrease. This was mainly the tool material. As the temperatures rose, the oxidation mass gain began to decrease. This was mainly This was mainly because high temperatures caused an oxide film to form, which separated the cutting because high temperatures caused an oxide film to form, which separated the cutting tool material from because high temperatures caused an oxide film to form, which separated the cutting tool material from tool material from oxygen. This made the oxidation reaction difficult. As the temperature increased, oxygen.This Thismade madethe theoxidation oxidationreaction reactiondifficult. difficult.As Asthe thetemperature temperatureincreased, increased,the thesublimation sublimationofofWO WO oxygen. 3 3 the sublimation of WO and CO gas escape caused the oxidation mass gain to decrease. 3 2 and CO 2 gas escape caused the oxidation mass gain to decrease. and CO2 gas escape caused the oxidation mass gain to decrease.
Theoxidation oxidationmass massgain gainofofthe thecoated coatedcarbide carbidetool tooloccurred occurredatat1100 1100KKwith witha asharp sharpincrease; increase;the thetrend trend The was similar to that of WC/Co. When the temperature rose to 1200 K, the oxidation mass gain continued was similar to that of WC/Co. When the temperature rose to 1200 K, the oxidation mass gain continued toto increaseand andeventually eventuallyexceeded exceededthat thatofofWC/Co. WC/Co.This Thiswas wasbecause becauseAlAlon onthe thecoating coatingsurface surfacewas wasoxidized oxidized increase 2O , whichattached attachedtotothe thetool toolsurface surfaceand andformed formedan anoxide oxidelayer layerwith witha agood goodprotective protectiveeffect effecton onthe the totoAlAl 2O 3, 3which tool matrix. The oxygen entered the tool matrix through the pores and defects of the coating as the tool matrix. The oxygen entered the tool matrix through the pores and defects of the coating as the
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The oxidation mass gain of the coated carbide tool occurred at 1100 K with a sharp increase; the trend was similar to that of WC/Co. When the temperature rose to 1200 K, the oxidation mass gain continued to increase and eventually exceeded that of WC/Co. This was because Al on the coating surface was oxidized to Al2 O3 , which attached to the tool surface and formed an oxide layer with a good protective effect on the tool matrix. The oxygen entered the tool matrix through the pores and Crystals 2018, 8, x FOR PEER REVIEW 6 of 13 defects of the coating as the temperature increased from 1100 K to 1200 K. This led to matrix oxidation and edge cracking of thethe oxide layer, which intensified Owing tothe thetool extensive layer, which intensified oxidation reaction. Owing tothe theoxidation extensivereaction. contact between matrix contact between the tool matrix material and oxygen, there was a significant increase in the oxidation material and oxygen, there was a significant increase in the oxidation mass gain. This was also the reason mass also the reason the large area shedding at the high temperature of 1273 K. for thegain. largeThis areawas shedding at the highfor temperature of 1273 K. The ceramic tool did not show an obvious oxidation mass gain The ceramic tool did not show an obvious oxidation mass gain below below 1100 1100 K, K, but but the the mass mass began began to slowly increase at 1100 K and above. As the tool matrix was Al O in the experiment, the ceramic 2 3 to slowly increase at 1100 K and above. As the tool matrix was Al2O3 in the experiment, the ceramic tool tool showed showed good good oxidation oxidation resistance. resistance. However, However, the the oxidation oxidation mass mass gain gain increased increased slowly slowly because because TiC in the ceramic tool was oxidized to TiO at high temperatures. TiC in the ceramic tool was oxidized to TiO22 at high temperatures.
Figure 5. 5. Oxidation mass gain curves curves of of different different tool tool materials materials versus versus temperature. temperature. (The (The insets insets Figure indicate the the changes changes in in physical physical form form and and colour colour of ofthe thetool toolmaterials). materials）. indicate
As As shown shown in in Figure Figure 5, 5, the theoxidation oxidationmass massgain gainof ofWC/Co WC/Co was not obvious obvious until until the the temperature temperature reached reached 973 973 K. K. Figure Figure 6a–d 6a–d shows shows SEM SEMimages imagesof ofthe theWC/Co WC/Co tool tool material material at at temperatures temperatures of of room room temperature, 873 K, K,1173 1173K, K,and and1273 1273K,K,respectively. respectively. compact oxidation layer appeared on tool the temperature, 873 AA compact oxidation layer appeared on the tool surface and totally covered it at 1173 K. When the temperature rose to 1273 K, the oxide particles surface and totally covered it at 1173 K. When the temperature rose to 1273 K, the oxide particles in the in the layer oxideincreased layer increased in size, and gaps obvious gaps appeared between theThere particles. were oxide in size, and obvious appeared between the particles. were There two reasons two reasons this.oxides First, some oxides were state in a gaseous state at high temperatures. example, for this. First,for some were in a gaseous at high temperatures. For example, For the oxidation the oxidation of WC gas. At 1100of K,solid the formation of solid product of WCproduct (WO3) had two(WO forms: solidtwo andforms: gas. Atsolid 1100and K, the formation WO3 was the main 3 ) had WO the main reaction. When temperature rose to31200 K, gaseous WO easily, and3 reaction. the temperature rosethe to 1200 K, gaseous WO formed easily, and the main form of WO 3 wasWhen 3 formed the formAs of the WOgaseous was gaseous. As the gaseous WO escaped, gaps appeared on the surface of the wasmain gaseous. WO 3 escaped, gaps appeared on the surface of the compact oxidation film. 3 3 compact oxidation film. Second, at thehigh solid WO3 sublimated high temperatures, which resulted in Second, the solid WO 3 sublimated temperatures, which at resulted in gaps appearing in the compact gaps in the compact oxidethe film. Oxygen thenthe penetrated the cutter to through the gap oxideappearing film. Oxygen then penetrated cutter through gap and continued react with the and tool continued to react with the material, which reduced products the tool performance. In addition, products material, which reduced the tool performance. In addition, such as WO3 and Co3O4 reacted with such as WO and Co3 OCoWO withCO oxygen CoWO with COgaps. oxygen and 3 produced 4 with 2, and and CO2produced escaped to also 4produce general, the 4 reacted 2 , andInCO 2 escaped to also produce gaps. Ingeneration general, the sublimation WO and generation of gaseous WO and CO sublimation of WO 3 and of gaseous WO3 of and CO 2 reduced the oxidation mass gain of the tool 3 3 2 reduced oxidation mass gain the tool mass material, coincided with 5. the oxidation mass gain material,the which coincided with theof oxidation gain which curve shown in Figure curveAccording shown in to Figure 5. 5, the oxidation mass gain of the coated carbide tool was not obvious before the Figure According 5, and the oxidation gain of the coated of carbide tool was notThe obvious before temperature rosetotoFigure 1100 K, there was mass no significant oxidation the tool material. tool material the temperature rose to 1100 K, and there was norose significant oxidation of the tool material. Theoftool showed obvious oxidation when the temperature to 1173 K. Figure 7a–d shows SEM images the material showed obvious oxidation when the temperature to 1273 1173K, K.respectively. Figure 7a–dAt shows SEM carbide tools at temperatures of room temperature, 973 K, 1173rose K, and 1173 K, the images thenot carbide tools atshed, temperatures ofremaining room temperature, 973 surface K, 1173 of K, the andtool 1273material. K, respectively. coatingof was completely with some on the oxide Loss of At K, the coating was not completely shed, with some on the rose oxide of the tool the1173 coating aggravated the oxidation of the tool material. Whenremaining the temperature to surface 1273 K, the coating of the tool material had completely fallen off, and the matrix of the tool material was fully oxidized. Gaps also appeared between the oxide granules for reasons similar to those described for the WC/Co tool material. As shown in Figure 5, the mass gain of the ceramic tool was not obvious at 973 K. The oxidation mass gain of the tool material increased slowly when the temperature rose to 1173 K. Figure 8a–d shows
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material. Loss of the coating aggravated the oxidation of the tool material. When the temperature rose to 1273 K, the coating of the tool material had completely fallen off, and the matrix of the tool material was fully oxidized. Gaps also appeared between the oxide granules for reasons similar to those described for the WC/Co tool material. As shown in Figure 5, the mass gain of the ceramic tool was not obvious at 973 K. The oxidation mass gain of the tool material increased slowly when the temperature rose to 1173 K. Figure 8a–d shows SEM images of the ceramic tool material at temperatures of room temperature, 973, 1173 K, and 1273 K, respectively. Figure 4 shows that the oxidation product TiO2 was observed at 1173 K, but part of the TiC was not oxidized. The TiC in the tool material completely oxidized to TiO2 when Crystals 2018, 8, x FOR PEER REVIEW 7 of 13 the temperature was increased to 1273 K. As shown in Figure 8, a large amount of oxidation product was found ontool the surface material at 973 K. Larger oxide on particles appeared thematerial surface surface of the materialofatthe 973tool K. Larger oxide particles appeared the surface of theon tool of the tool material at 1173 K. At 1273 K, the tool oxidation products continued to increase, and the at 1173 K. At 1273 K, the tool oxidation products continued to increase, and the surface was covered surface was covered with oxidized particles. with oxidized particles.
(a)
(b)
(c)
(d)
Figure Figure 6. 6. Oxidation Oxidation morphology morphology of of WC/Co WC/Coatatdifferent differenttemperatures: temperatures:(a) (a)room roomtemperature temperature(b) (b)873 873K, K, (c) 1173 1173 K, K, and and (d) (d)1273 1273K. K.
(a)
(b)
(c)
(d)
Crystals 2018, 8,6.471 Figure Oxidation morphology of WC/Co at different temperatures: (a) room temperature (b) 873 K,8 of 14
(c) 1173 K, and (d) 1273 K.
(a)
(b)
(c)
(d)
Figure 7.x FOR Oxidation morphology at different different temperatures: temperatures: (a) room room Figure Oxidation morphology of of the coated carbide tool at Crystals 2018, 8,7. PEER REVIEW 8 of 13 temperature (b) (b) 873 873 K, K, (c) (c) 1173 1173 K, K, and and (d) (d) 1273 1273 K. K. temperature
(a)
(b)
(c)
(d)
Figure 8. Oxidation morphology of the ceramic tool at different temperatures: (a) room temperature (b) 873 K, (c) 1173 K, and (d) 1273 K.
The above analysis indicates that the different kinds of tool materials showed different degrees of oxidation at high temperatures. The ceramic tool material had the lowest degree of oxidation. The WC/Co tool material showed slight oxidation at 973 K and very intense oxidation at 1173 K. The coated carbide tool material showed an obvious oxide layer at 1173 K, and the coating started to chip. At 1273 K, the coating of the cutter was chipped over a large area. In conclusion, the ceramic tool material showed the strongest antioxidant properties, followed by the coated carbide and then WC/Co. Severe
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The above analysis indicates that the different kinds of tool materials showed different degrees of oxidation at high temperatures. The ceramic tool material had the lowest degree of oxidation. The WC/Co tool material showed slight oxidation at 973 K and very intense oxidation at 1173 K. The coated carbide tool material showed an obvious oxide layer at 1173 K, and the coating started to chip. At 1273 K, the coating of the cutter was chipped over a large area. In conclusion, the ceramic tool material showed the strongest antioxidant properties, followed by the coated carbide and then WC/Co. Severe oxidation of tool materials will result in a rapid deterioration in material properties [15]. 3.2. Diffusion Behaviour of the Tool Material 3.2.1. Elemental Diffusion Mechanism According to Fink’s laws of diffusion, elements from two tightly attached objects diffuse to some degree from the object with the higher concentration to the one with the lower concentration. During the experimental process, which was limited to 90 min, the concentration at each point changed with the diffusion distance. This was used to set diffusion models according to Fink’s second law of diffusion: C x C ( x, t) = 1 1 − er f √ (2) 2 2 Dt where C represents the concentration of the diffusion element (kg/m3 ), C1 represents the initial concentration of the diffusion element (kg/m3 ), t represents the diffusion time (s), x represents the diffusion distance (m), and D is the diffusion coefficient (m2 /s). Equation (2) gives the relationship between the concentration and distance of diffusion elements so that the diffusion concentration can be obtained different distances. Crystals 2018, 8,at x FOR PEER REVIEW 9 of 13 3.2.2. Analysis of of the the Diffusion Diffusion Experiment 3.2.2. Analysis Experiment To To verify verify that that the the experimental experimental results results were were satisfactory, satisfactory, an an analysis analysis was was performed performed on on the the line line scans of the tool and workpiece materials shown in Figure 9, and the XRD patterns of the tool material scans of the tool and workpiece materials shown in Figure 9, and the XRD patterns of the tool material shown As shown shown in in Figure shown in in Figure Figure 10. 10. As Figure 9a–c, 9a–c, the the elements elements in in the the three three tool tool materials materials had had different different degrees of diffusion at 873 K. Diffusive reactions also occurred at lower temperatures. degrees of diffusion at 873 K. Diffusive reactions also occurred at lower temperatures. As As shown shown in in Figure of the Figure 10, 10, the the material material composition composition of the tool tool showed showed almost almost no no change change at at 1273 1273 K. K. The The oxidation oxidation simulation that more significant oxidation reaction phenomena occurred at higher simulation experiments experimentsshowed showed that more significant oxidation reaction phenomena occurred at temperatures and that the three tool materials diffused at various temperatures. No oxidation reaction higher temperatures and that the three tool materials diffused at various temperatures. No oxidation occurred during the experiment, and the experiment reached the expected state. reaction occurred during the experiment, and the experiment reached the expected state.
(a)
(b)
(c)
Figure 9. Line scan of the tool materials materials and and Inconel Inconel 625 625 at at 873 873 K. K. (a) (a) WC/Co, WC/Co, (b) coated carbide, and (c) ceramic.
(a)
(b)
(c)
Figure 9. Line scan of the tool materials and Inconel 625 at 873 K. (a) WC/Co, (b) coated carbide, and (c) ceramic. Crystals 2018, 8, 471
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(a)
(b)
(c)
Figure10. 10.XRD XRDpatterns patternsof ofthe thetool toolmaterial materialatat1273 1273K. K.(a) (a)WC/Co, WC/Co, (b) (b) coated coated carbide, carbide,and and(c) (c)ceramic. ceramic. Figure
In In order order to toquantify quantify the theelement element diffusion diffusion behaviour, behaviour, EDS EDS was was used usedto toanalyze analyze the thediffusion diffusion element concentration at points on both sides of the contact interface. The diffusion element concentration at points on both sides of the contact interface. The diffusion element element concentrations used to draw diffusion curves. Figure 11 shows the element diffusion curves concentrationswere werethen then used to draw diffusion curves. Figure 11 shows the element diffusion of Inconel 625 and 625 the and threethe tool materials at temperatures of 873 K,of1173 K. Figure was curves of Inconel three tool materials at temperatures 873 K, and 11731273 K, and 1273 K.11 Figure scanned by Figureby 9. Figure As shown in Figure b and g,11a, thebintensity of the diffusion main element 11 was scanned 9. As shown11a, in Figure and g, the intensity of of thethe diffusion of the W of WC/Co Inconel 625into significantly increased with increasing as did the intensity main elementinto W of WC/Co Inconel 625 significantly increasedtemperature, with increasing temperature, as of diffusion of the main elements Ni and Cr from Inconel 625 to WC/Co. As shown in Figure 11b, e, did the intensity of diffusion of the main elements Ni and Cr from Inconel 625 to WC/Co. As shown and h, the diffusion of h, thethe main elements Al and Nielements of the coated carbide Inconel 625 became in Figure 11b, e, and diffusion of the main Al and Ni oftool the to coated carbide tool to significant the temperature while the elements and Cr less in the Inconel 625asbecame significantincreased, as the temperature increased,Tiwhile theshowed elements Tivariation and Cr showed degree of diffusion. As shown in Figure 11c, f and i, the diffusion of elements between the ceramic tool less variation in the degree of diffusion. As shown in Figure 11c, f and i, the diffusion of elements and Inconel was very at the different Comparisons showed that the elemental between the625 ceramic toolweak and Inconel 625 was temperatures. very weak at the different temperatures. Comparisons diffusion thethe different materials variedof drastically at different temperatures. The temperature had showed of that elemental diffusion the different materials varied drastically at different the greatest effect the diffusion of WC/Co, by that of the of coated carbide and temperatures. Theon temperature hadintensity the greatest effect onfollowed the diffusion intensity WC/Co, followed then thatofofthe thecoated ceramic. by that carbide and then that of the ceramic. According Accordingto tothe thediffusion diffusiondistances distancesof ofInconel Inconel625 625with withthe thethree threetool toolmaterials, materials,the theceramic ceramictool tool showed Forexample, example,Figure Figure11i 11ishows showsthe thediffusion diffusion the elements showed excellent excellent diffusion inhibition. For ofof the elements Al Al and ceramic tool into Inconel at 1273 diffusion distance about 5 µm. and Ti Ti of of thethe ceramic tool into Inconel 625625 at 1273 K. K. TheThe diffusion distance waswas about 5 µm. Ni Ni diffused from Inconel 625 to the ceramic tool, and the diffusion distance was about 10 µm. Al and Ti in the coated carbide tool diffused into Inconel 625, and the diffusion distance was about 10 µm. Ni diffused from Inconel 625 into the coated carbide tool, and the diffusion distance was about 15 µm. The diffusion distances of Ni and W between WC/Co and Inconel 625 both reached 20 µm. The collected data shown in Figure 11 were used to obtain the initial concentrations of the elements and the concentrations of each diffusion element. The diffusion coefficients were calculated using the diffusion model. Tables 5–7 present the diffusion coefficients of the main elements in the three tool materials and Inconel 625. Compared with the other two tools, the diffusion coefficient of the WC/Co tool was most affected by the temperature. Table 5 indicates that the diffusion coefficients of the main elements of WC/Co and Inconel 625 increased significantly with increasing temperature. For example, the diffusion coefficient for Cr increased from 4.04 × 10−15 m2 /s to 1.32 × 10−14 m2 /s between 873 K and 1273 K. The diffusion coefficients of each element also differed at the same temperature. For example, at 1273 K, W had a smaller diffusion coefficient than any of Cr, Co, and Ni, so it had a smaller diffusion capacity. In Tables 6 and 7, the diffusion coefficients of the coated carbide and ceramic tools changed less as the temperature increased. In the case of the coated carbide tool, the surface coating prevented the diffusion of elements, resulting in lower diffusion coefficients than those of the WC/Co tool at the same temperature. The diffusion coefficient of Cr in the coated carbide tool was most affected by the temperature. Between 873 K and 1273 K, the diffusion coefficient increased from 9.85 × 10−16 m2 /s to 1.63 × 10−15 m2 /s. The diffusion coefficients of the ceramic tool increased slowly with increasing temperature. In addition, each element of the ceramic tool had a small diffusion coefficient. Cr had a larger diffusion coefficient than the other three elements, but it was still only 1.46× 10−15 m2 /s at 1273 K.
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diffused from Inconel 625 to the ceramic tool, and the diffusion distance was about 10 µm. Al and Ti in the coated carbide tool diffused into Inconel 625, and the diffusion distance was about 10 µm. Ni diffused from Inconel 625 into the coated carbide tool, and the diffusion distance was about 15 Crystals 2018, 8, 471 11 µm. of 14 The diffusion distances of Ni and W between WC/Co and Inconel 625 both reached 20 µm.
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(a)
Figure Inconel 625. (a) (a) WC/Co (973(973 K), (b) Figure 11. 11. Element Elementdiffusion diffusioncurves curvesbetween betweentool toolmaterial materialand and Inconel 625. WC/Co K), coated carbide (973 K), (c) ceramic (973 K), (d) WC/Co (1173 K), (e) coated carbide (1173 K), (f) ceramic (b) coated carbide (973 K), (c) ceramic (973 K), (d) WC/Co (1173 K), (e) coated carbide (1173 K), (1173 K), (g)(1173 WC/Co K), (h) coated carbide (1273 K), and (i) ceramic (1273 (Error(1273 bars are (f) ceramic K),(1273 (g) WC/Co (1273 K), (h) coated carbide (1273 K), and (i)K). ceramic K). smaller thanare the symbols.) (Error bars smaller than the symbols.) Table 5. Diffusion coefficients of the11 main elements (90 min). The collected data shown in Figure were used of toWC/Co obtain and the Inconel initial 625 concentrations of the 2 elements and the concentrations of each diffusion element. The diffusion coefficients were calculated Diffusion Coefficient (m /s) Diffusion Temperature using the diffusion model. Tables diffusion coefficients in the W 5–7 present the Cr Co of the main elements Ni −15 15 three tool 873 materials and Inconel 625. the two±tools, K (4.35 ± 0.42) × 10Compared (4.04 ±with 0.35) × 10−other (4.96 0.49) ×the 10−15diffusion (2.12 ±coefficient 0.79) × 10−15 of −15 −14 1173 K was most(6.02 ± 0.69) by × 10the (9.13 ± 0.76) ×Table 10−15 5 indicates (1.36 ± 0.31)that × 10the (6.79 ± 0.35) × 10−15 the WC/Co tool affected temperature. diffusion coefficients 1273 K (9.91 ± 1.11) × 10−15 (1.32 ± 0.53) × 10−14 (1.79 ± 0.59) × 10−14 (1.78 ± 0.30) × 10−14 of the main elements of WC/Co and Inconel 625 increased significantly with increasing temperature. For example, the diffusion coefficient for Cr increased from 4.04 × 10−15 m²/s to 1.32 × 10−14 m²/s between Table 6. Diffusion coefficients of the main elements of the coated carbide and Inconel 625 (90 min). 873 K and 1273 K. The diffusion coefficients of each element also differed at the same temperature. Coefficient (m2 /s)of Cr, Co, and Ni, so it had a ForDiffusion example, at 1273 K, W had a smaller diffusion Diffusion coefficient than any Temperature Cr Ni of the coated carbide Ti smaller diffusion capacity. In Tables 6 and 7, theAldiffusion coefficients and −16 −15 −15 −15 873 K (9.85 ± 0.79) × 10 (1.52 ± 0.26) × 10 (1.28 ± 0.11) × 10 (1.23 ± 0.12) × 10 ceramic tools changed less as the temperature increased. In the case of the −coated carbide tool, the 1173 K (7.85 ± 1.21) × 10−16 (1.85 ± 0.02) × 10−15 (1.70 ± 0.18) × 10 15 (1.13 ± 0.15) × 10−15 − 15 − 15 − 15 15 surface coating prevented the diffusion of elements, resulting in lower diffusion coefficients 1273 K (1.63 ± 0.06) × 10 (2.34 ± 0.25) × 10 (1.78 ± 0.21) × 10 (2.40 ± 0.31) × 10−than
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Table 7. Diffusion coefficients of the main elements of the ceramic tools and Inconel 625 (90 min). Diffusion Coefficient (m2 /s)
Diffusion Temperature 873 K 1173 K 1273 K
Al
Ti
Ni
Cr
(7.91 ± 1.25) × 10−16 (8.08 ± 1.23) × 10−16 (1.02 ± 0.05) × 10−15
(9.9 ± 2.10) × 10−16 (1.26 ± 0.02) × 10−15 (1.56 ± 0.04) × 10−15
(9.87 ± 1.19) × 10−16 (9.58 ± 1.45) × 10−16 (9.94 ± 2.12) × 10−15
(1.35 ± 0.12) × 10−15 (1.35 ± 0.12) × 10−15 (1.46 ± 0.25) × 10−15
W and Ni are the main components of WC/Co and Inconel 625, respectively, and changes in their concentrations affected the mechanical properties of the tool and the alloy. At the same time, W and Ni were the two elements that showed the highest degree of diffusion. When element diffusion occurred in WC/Co and Inconel 625, W had a significantly higher initial concentration than the other three elements (Cr, Co, and Ni), so the diffusion concentration of W was greater than those of the others at each temperature. When the temperature was increased from 873 K to 1173 K, the Ni atoms moved rapidly, and the diffusion concentration value gradually increased. When the temperature was increased to 1273 K, the activity intensities of the W and Ni atoms increased even further, and the original concentrations of W and Ni were relatively high. Therefore, the W and Ni elemental diffusion concentrations clearly increased. Owing to the low initial concentrations of Co and Cr, their diffusion concentrations were not obvious, but their diffusion coefficients were similar to those of the other two elements. The coated carbide tool and Inconel 625 had similar element diffusion tendencies to that of WC/Co, but the intensity of the element diffusion was not significant. This is because the coating provided good protection and obstruction, which prevented the tool and workpiece elements from diffusing. As the temperature increased, the coating spalled, which reduced its resistance to element diffusion. Thus, the degree of diffusion of Al and Ni began to increase. Although Ti and Cr had relatively small concentrations, the increase in element diffusion was not obviously connected with temperature. However, the diffusion coefficients indicated that the two elements had greater diffusion than the other elements. The ceramic tool and Inconel 625 had the weakest degree of diffusion. This was because the Al2 O3 barrier layer formed a defect-free hcp crystal structure. In such a structure, the atomic arrangement is tight and orderly, and there is no phase change when the temperature is increased to its melting point. It also has excellent bonding strength and can effectively inhibit element diffusion [16]. In summary, the elemental diffusion intensity of Inconel 625 and the three tool materials were quite different. The elemental diffusion intensity was highest with WC/Co, followed by the coated carbide and then the ceramic. Element diffusion between a cutting tool and the workpiece material leads to composition change of the tool, which may increase the possibility of mechanical damage to the tool [17]. In terms of diffusion, the ceramic material is preferable for cutting Inconel 625. 4. Conclusions Oxidation and diffusion experiments were carried out on Inconel 625 with three tool materials (WC/Co, coated carbide and ceramic). The results led to the following conclusions: (1) For WC/Co the oxidation reaction occurred at 973 K, with WC and Co being oxidized to WO3 and Co3 O4 as well as the common product CoWO4 . As the temperature was increased, the oxidation behaviour became more intense. WO3 sublimated, so the oxidation mass gain increased before decreasing. The coated carbide tool showed obvious oxidation behaviour at 1173 K, and it started to shed its coating. When the temperature rose to 1273 K, a large area of the tool material showed shedding. The ceramic tool material showed good oxidation resistance with almost no oxidation mass gain. TiC was oxidized only to TiO2 . (2) The diffusion capacities of the three tool materials and the superalloy Inconel 625 increased with temperature. The main elements affected were W and Ni. The diffusion coefficient of W was very large, and the diffusion capacity of Ni increased with temperature. Because of the hindering effect of the coating on element diffusion and the closed structure of the ceramic tool material (Al2 O3 ),
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neither these two tool materials nor Inconel 625 showed any obvious diffusion. The experimental results showed that the ceramic tool material had the highest capacity for diffusion resistance. (3) In terms of oxidation and diffusion, the ceramic material is preferable for cutting Inconel 625, followed by the coated carbide and then YG8. Author Contributions: Conceptualization, E.L. and N.W.; methodology, Z.X. and X.L.; data curation, H.Z. and J.Q.; writing—original draft preparation, E.L. and N.W.; writing—review and editing, N.W. Funding: This research was funded by the National Natural Science Foundation of China (Granted Number: 51475126), the Youth Science and Technology Innovation Talents Foundation of Harbin (Granted Number: 2016RAQXJ001). Acknowledgments: This work was supported by the National Natural Science Foundation of China (Granted Number: 51575145). Conflicts of Interest: The authors declare no conflict of interest.
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