ISSN 10683666, Journal of Friction and Wear, 2014, Vol. 35, No. 6, pp. 470–476. © Allerton Press, Inc., 2014. Original Russian Text © I. Smurov, M. Doubenskaia, S.N. Grigoriev, D.V. Kotoban, P.A. Podrabinnik, 2014, published in Trenie i Iznos, 2014, Vol. 35, No. 6, pp. 682–690.
Application of Laser Surface Engineering to Solve Tribological Problems I. Smurova, *, M. Doubenskaiaa, S. N. Grigorievb, D. V. Kotobanb, and P. A. Podrabinnikb a
Université de Lyon, Ecole Nationale d’Ingénieurs de Saint Etienne, DIPI Laboratory, 58 rue Jean Parot, 42023 SaintÉtienne Cedex 2, France bMoscow State Technological University Stankin, Vadkovskii per. 1, Moscow, 127994 Russia *email:
[email protected] Received August 8, 2014
Abstract—The methods of laser surface engineering, including laser cladding, laser alloying from predepos ited layers, and laser alloying from the gas phase have been applied to increase the wear resistance and decrease the coefficient of friction of metallic surfaces. Laser cladding has been applied to deposit metal matrix composites that consist of CuSn, stellite, stainless steel, and bronze, which act as solid lubricant rein forced with nanostructured WC ceramics with Co binding. Laser alloying of bearing steel with Sn increases the lifetime of oil turbo drills. Wear tests of Ti alloys doped with nitrogen from a gas atmosphere have demon strated a viable possibility to apply the developed method to fabricate frictional couples. Keywords: laser cladding, laser alloying, coefficient of friction, wear intensity, metal matrix composite materials DOI: 10.3103/S1068366614060129
INTRODUCTION
Laser Cladding and Direct Laser Deposition of Metal
Currently, various methods of laser surface engi neering have been developed, including hardening, alloying, cladding, cleaning, fabrication of pits and channels for lubrication transport, etc. [1, 2]. This article discusses the following method of laser surface engineering: laser cladding, alloying from predepos ited layers and coatings, and alloying from a gas phase. These methods are applied in order to improve tribo logical surface properties: increase in microhardness and wear resistance, a decrease in the coefficient of friction, etc.
Laser cladding is a flexible and efficient method of obtaining various protective coatings, including func tionally graded and multilayer coatings. It is possible to mix various powders within their addition to the cladding area and to obtain coatings with the required composition and internal structure [6–9].
Typically, the following methods are used to add dopants to the area of interactions between the laser beam and substrate: additive powder and predeposited coatings from the gas (plasma) phase and from the layer of transparent liquid [3–5]. As a rule, these methods are not very efficient within laser alloying, when it is required to achieve good mixing of compo nents with various physical properties. This article proposes several variants for solving this problem, including the use of protective shields that promote the elimination of destruction and the evap oration of the doping element from substrate surface and the preliminary deposition of thick (more than 100 μm) multilayer coatings of doping components, as well as repetitively pulsed laser impact.
In socalled coaxial laser cladding, a mixture of powders is transported to the substrate coaxially to the laser beam. The advantages of coaxial cladding head include the possibility of 3D cladding and the protec tion of the powder from the environment, as well as a relatively moderate area of thermal action. Laser clad ding can be applied to fabricate a protective layer in any selected location, which is exposed to significant wear, or to restore locally damaged or worn surfaces. Robotic laser cladding with coaxial feeding of pow der and integrated optical diagnostics is often referred to as direct metal deposition (DMD), which is applied in industries such as aircraft engineering, the automo tive industry, and biomedicine [10–15]. Laser Doping from Predeposited Layers The technology is based on mixing predeposited layers with the base material and fabricating the near surface layer with the required composition and prop erties [1, 6].
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Properties of coatings obtained by laser cladding Coating composition
Average microhard ness, HV
Average coefficient of friction
Remarks
0.32
Stellite as matrix was used with addition of CuSn as solid lubricant. Laser irradiation mode was continuous
0.4
Stellite as matrix was used with inclusions of CuSn; nanostructured WC–Co was used to increase wear resistance. Laser irradiation mode was continuous
730/480
0.12
Stellite as matrix was used (reinforcing walls) and CuSn; nanostructured WC–Co was used to increase wear resistance. Laser irradiation mode was repetitively pulsed in order to improve mixing of components
SS 430L/CuSn 70/30 vol %
500
0.41
Stainless steel was used as the matrix, CuSn was solid lubricant. Laser irradiation mode was continuous
Bronze/WC–Co/Cr 50/50 vol %
280
0.45
Bronze used as solid lubricant enforced with WC–Co/Cr. Laser irradiation mode was con tinuous
Bronze/WC–Co/Cr 50/50 vol %
300
0.25
Bronze used as solid lubricant enforced with WC–Co/Cr. Laser irradiation mode was repet itively pulsed in order to improve mixing of components
Stellite/CuSn 70/30 vol %
Stellite/CuSn/nanostructured WC–Co 56/24/20 vol %
Stellite/CuSn/nanostructured WC–Co 56/24/20 vol %
500
930/530
Another method of laser doping is doping from the gas phase. In this case, the doping element from the gas phase is absorbed by the base material fused with laser irradiation [1, 6]. Currently, numerous methods are applied to solve the wear problem; however, there are still plenty of unsolved applied problems related to reliability and long operation lifetime. For instance, there are no acceptable methods for improving the wear resistance of friction units, which operate under difficult condi tions, including high loads and high contact pressure, lubricant being washed out of the contact area, aggres sive corrosive environment, high temperatures, etc. In some cases, the interaction between friction parts occurs in dry mode. This leads to intensive linear wear and rapid destruction of mechanical parts. The wear of friction units of oilproduction equipment (bearings, seals, valves, etc.) is a typical example of the high lighted problem. For instance, long operation lifetime of multirow bearings of turbo drills manufactured by conventional methods is only 4–6 h. Tribotechnical joints that operate under severe conditions have high wear allowance ranging from several hundred microns to several millimeters. Therefore, the thickness of modified layer should be comparable with the wear allowance. JOURNAL OF FRICTION AND WEAR
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The aim of the work was to fabricate of thin nitride layer about 10 μm thick, as well as a substrate layer of nitrogen solid solution in titanium. This structure is better processed by plastic deformation, since the solid layer of titanium nitride is supported by a plastic solid phase layer that prevents crack formation. MATERIALS AND METHODS The experiments were carried out using two facili ties, including (a) TrumaForm DMD 505 Trumpf (CO2 laser, maximum power 5 kW, continuous mode of laser irradiation) equipped with the feeding system for two different powders and (b) a fiveaxis table with digital control LASMA 1054 supplied by TRUMPF (Nd:YAG HAAS 2006D laser, maximum power was 2 kW, operation mode was continuous and repetitively pulsed): ⎯Powder feeding was carried out with a MEDI COAT dualchannel antistatic feeder capable of vary ing the powder flow rate separately for each channel. Argon was used as the carrier gas. ⎯Three coaxial cladding heads of different designs were applied. The protective gas (argon) focused the powder and surrounded (thus protecting against oxi dation) the powder and the cladding area. 2014
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⎯Typical parameters of the laser cladding an power density of 5 × 103 W/cm2, a beam scanning speed of 5–15 mm/s, a powder flow rate of 0.3– 0.5 g/s, and a width of the clad bead of 1–7 mm. The particle size distribution of the powder was determined using an ALPAGA 500 NANO (OCCHIO s.a.) system for granulomorphometric analysis with software capable of analyzing the image and acquiring data on the size of the particle and shape of the powder. The stage of powder preparation includes drying in furnace, as well as a powder sieving RETSCH machine to obtain a powder with the predetermined particle size. The range of the applied sieves varied from 100 to 20 μm. Powder mixtures were agitated in Bioengineering Inversina 2I mixer for 15 min. The study involved the use of CO2 laser, capacity 2.5 kW, and Nd:YAG repetitively pulsed laser (pulse duration τ = 1–10 ms, pulse energy E = 1–30 J). In order to protect metal against oxidation, argon was used during processing at a flow rate of 50 L/min directed coaxially with the laser beam. The steel used for specimens and bearing races had the following composition: 0.5% C, 0.8–1.0% Si, 0.5–0.6% Mo, and 0.1% V. A coating made of tin was covered with a galvanic protective coating of chromium. This twolayer system was exposed to laser impact under optimum process ing parameters, which lead to the intensive mixing of tin and steel at a depth of up to 500 μm. Testing under the conditions of sliding friction (dry friction and with oil lubrication) were performed using a pinondisk machine. Flat surfaces of the disk were alloyed with tin (Sn) using an Nd:YAG repeti tively pulsed laser. The tests were performed under the following conditions: ⎯the pin material was highalloy steel with a hard ness of 60 HRC and 6 mm in diameter; ⎯loading was 2 MPa for dry friction and 4 MPa for friction with oil lubrication; ⎯sliding speed was 0.2 m/s for dry friction and 0.8 m/s was with oil lubrication; ⎯the duration of the test was 1 h; ⎯lubrication was paraffin oil. Coefficient of friction was the main measured parameter. Wear rate and operation lifetime of multirow bear ing races of turbo drill were studied after doping using CO2 laser. The tests were performed under conditions most closely resembling those within operation: axial loading F = 15 kN, contact pressure σ = 2500 MPa, rotation frequency of ω = 700 min–1, external condi tions: water temperature T = 25°C. Two disks were fabricated of titanium alloy VT23 with the following parameters: the diameters were 38 and 10 mm and the thicknesses were 10 and 3 mm, respectively. The values of roughness R were initially
0.78–1.05 μm. The surface roughness was measured using Talysurf 6 instrument in accordance with the regulations IS0 4287. The disks were processed using repetitively pulsed laser with wavelength 1.06 μm and pulse duration 5 ms. The power density of laser irradi ation varied in the range of 2–3 GW m–2. In this proce dure, nitrogen was fed into the processing area coaxially with a laser beam. The gas flow rate was 40 L min–1. Plastic deformation was carried out with a special hard cylindrical roller using the machine; the load was 200 kg and the rotation velocity was 100 rpm. The specimens were tested for wear according to the diskondisk flowchart. The tests were carried out as follows: prior to weighing, the disks were cleaned with alcohol and dried in a furnace at 100°C. The tests were carried out in water at 27°C, both disks rotated in opposite directions at a contact pressure 400 MPa, rotation frequency n = 1000 rpm, and sliding speed V = 0.4 m/s. After each test, the disks were cleaned in alcohol and dried in a furnace at 100°C; then, the dif ferences in diameter and weight were measured. A total of 20 tests were performed, and the duration of each test was 4 h. The worn surfaces were analyzed using SEM. The main parameter of wear is its intensity I, which is defined as the ratio between linear wear and friction distance s as shown below:
I = δ s.
(1)
Linear wear is defined as the total decrease in the diameter of the rotating disks due to wear. The friction distance s corresponds to the area of elastic contact between two disks and is derived from the following equation: Vs (2) n t, V1.2 1.2 where qn is the average normal loading per unit contact length, θ1 and θ2 are the elastic constants of disk mate rials, ρ is the relative radius of curvature of two disks, Vs is the sliding speed, V1,2 is the disk rolling speed, n1,2 is the disk rotation frequency, and t is the test time. s = 2.25 [qn ( θ1 + θ 2 ) ρ]
1/2
RESULTS AND DISCUSSION Laser Cladding The two following types of coating structures have been developed and tested: (1) Multifunctional coatings that provide a low coefficient of friction and increased wear resistance. A composite coating based on a metallic matrix that acts as solid lubricant consists of, e.g, CuSn, and is enforced with a ceramic phase, e.g., nanostructured WC/Co, as is schematically presented in Fig. 1a. An experimental specimen with this coating is shown in Fig. 1b, where CuSn is applied as solid lubricating plastic metallic matrix enforced with WC/Co 40 vol %.
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Reinforcing phase SiC WC/Co WC/Co/Cr
Substrate Z6NCTD 25, Z20, CNWS 211203
(a)
(b)
250 µm
Fig. 1. (a) Flowchart of composite coating based on metallic matrix acting as solid lubricant enforced with ceramic inclusions and (b) transverse cross section of composite coating based on solid lubricant matrix: CuSn (60 vol %) and reinforcing component WC/Co (40 vol %).
0.59 mm
Reinforcing phase Plastic matrix WC/Co (solid lubricant) CuSn 0.49 mm
Firm wear resistant core Stellite Substrate
Z6NCTD 25, Z20, CNWS 211203
(a)
0.1 mm
(b)
Fig. 2. (a) Flowchart of composite coating based on metallic matrix acting as solid lubricant enforced with ceramic inclusions and solid walls and (b) transverse cross section of composite coating based on solid lubricant matrix: CuSn (60 vol %) and reinforcing component WC/Co (40 vol %) with firm internal walls (stellite).
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1 mm/s, displacement of 5 mm, duration of 360 cycles (1 h), and temperature of 25°C. The coefficient of friction is presented in Fig. 3 as a function of the num ber of cycles. The stellite/CuSn/nanoWC/Co specimen (see cross section in Fig. 2b) is characterized by a steady coefficient of dry friction of 0.12, which is the lowest value among the tested coatings. The experimental results for the coatings are sum marized in Table 1. Microhardness was measured with a load of 300 g and rounded off. For several coatings, microhardness is characterized by two values, the first Coefficient of friction
(2) A coating with an internal architecture that consists of solid walls made from, e.g., stellite, located at a certain distance from each other. The free space between these walls is filled with solid lubricant, e.g., CuSn, which is enforced with a disperse ceramic phase. This coating is schematically illustrated in Fig. 2a. The firm internal core bears definite func tional loading, i.e., the reinforcement of the body in order to provide resistance against horizontal dis placement by the counterbody; a concentrator for plastic solid lubricant in order to prevent its removal; and an obstacle to possible crack propagation, which makes it possible to increase the operational loading of the contact. When coaxial cladding head and powder components of core and solid lubricant are applied, such coating can be achieved in one step. Each inter nal wall is formed by corresponding pass of coaxial cladding head; Fig. 2a shows three lines made by three passes of nozzle. Transversal cross section of a speci men with this coating is illustrated in Fig. 2b. Firm internal walls are made of stellite and free space between them is filled with CuSn, enforced with WC/Co. Coatings obtained in optimum modes are charac terized by low porosity (less than 2%) and the absence of cracks and minimum mixing with the substrate. Tribological tests were performed with these coat ings. Test modes were as follows: loading at 500 N (resulting pressure was 10 MPa), a sliding speed of
0.8 0.6 1
0.4
2 0.2 3 0 1
41
81 121 161 201 241 281 321 361 Cycles
Fig. 3. Variations in coefficient of friction within reciprocating tests: (1) steel 430L /CuSn /WCCoCr 70/25/5 vol %; (2) steel 430L /CuSn /nanoWCCo 73/20/7 vol %; (3) Stellite /CuSn/ nanoWC 56/24/20 wt %. 2014
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0.54 Coefficient of friction
Coefficient of friction
0.54 0.48 0.32 0.16
0
5 10 15 20 25 30 35 40 45 50 Time, min
(b)
0.48 0.32 0.16
0
5 10 15 20 25 30 35 40 45 50 Time, min
Fig. 4. Coefficient of friction of bearing steel alloyed with tin using repetitively pulsed Nd:YAG laser as a function of time within wear tests on pinondisk machine: (a) sliding within dry friction and (b) sliding with oil lubrication.
of which corresponds to core material, while the sec ond corresponds to solid lubricant. Alloying from Predeposited Layers The main purpose of tests using a pinondisk machine was to study doped layers under dry friction. In this case, at the start of the tests, a quasistationary value of the coefficient of friction was observed (Fig. 4a). The average value of the coefficient of friction (0.23) is low enough to avoid abrasion and adhesive wear for 1 h. The friction mechanism is as follows: tin reaches the contact surface in the form of intermetallic compounds or solid solution, either in the form of 1.0 1
Wear intensity, mm/h
0.8
0.6
0.4 2 0.2
0
4
8 12 Time, h
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minor inclusions. The steady state of the coefficient of friction can be attributed to the intensive transfer of tin (Sn) from one point of the disk surface to another. This keeps the coefficient of friction at a low level due to the solidlubricant properties of tin (Sn). A wear test with the application of oil lubricant under the conditions of increased loading and sliding speed leads to the total wear of the modified layer after 30 min. As a conse quence, the coefficient of friction rapidly increases from a low value (0.08) to 0.45 (Fig. 4b). Based on the results of the aforementioned tests, optimum parameters of laser doping were selected for processing internal and external bearing races of turbo drill. Initial (short) stage of tests for bearing wear (Fig. 5, curve 2) corresponds to the grinding of the parts. Steadiness of wear rate (the second stage) is pro vided by constant refeeding of solid lubricant from doped layers and constant intensive transfer of Sn between friction members. Geometry of laser tracks (width, thickness and position on bearing races) was selected so that to provide the best lubrication in the ranges of wear allowance. This protects the friction surface against intensive wear and destruction. Drastic increase in wear rate is observed only after complete wear of the areas containing tin. Compari son with regular method of induction hardening (Fig. 5, curve 1) demonstrated increase in operation lifetime by 2.5 times and decrease of wear rate by 3 times. In the case of hardening of external and inter nal races by high frequency currents complete destruc tion of bearing is observed after 6 h of wear test. At the same time the bearing on which the surface layer had been exposed to laser doping remained functional.
20
Alloying from the Gas Phase
Fig. 5. Wear intensity as a function of time for bearing races of turbo drill: (1) induction hardening; (2) laser alloying with tin.
A metallographic study of polished cross sections of specimens processed with a laser revealed white and dark areas with sharp boundaries. The white areas had a den
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4
3 15
Wear intensity
Nitrogen, wt %
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2 10
3
2
5 1 0
10–8 0
50
100 Depth, µm
Fig. 6. Nitrogen distribution in transverse cross section of titanium alloy after laser processing in nitrogen flow within (1) one, (2) two, and (3) three pulses.
CONCLUSIONS Coaxial laser cladding is a technology that enables the fabrication of various composite coatings with metallic matrices that have improved properties by introducing strengthening additives. A promising approach is the application of wear resistant coatings. For instance, a coating based on the metallic matrix, Vol. 35
7.2 10.8 Testing time, 103 s
14.4
Fig. 7. Wear intensity as a function of time for VT23 tita nium alloy after nitrogenation from gas phase.
drite structure with an axis that corresponded to the direction of the temperature gradient at the cooling stage. In this case, the Vickers microhardness measured under a load of 100 g was in the range of (10–12) × 103 MPa. The thickness of the dark areas was 150 μm, and their microhardness was in the range of (7–8) × 103 MPa. The Auger microanalysis of the cross section of the dark area revealed a sharp decrease in nitrogen con centration over the depth (Fig. 6, curve 1). Within measurements of the nitrogen concentration, a maxi mum value of about 22 wt % was obtained on the sur face, which corresponds to TiN. The value at the first point on the cross section was only 16–17%. There fore, it is possible to conclude that the thickness of the layer of pure titanium nitride is less than 5 μm. It is possible to apply multipulse laser processing in order to increase the nitrogen concentration in the nearsur face layer (Fig. 6, curves 2, 3). Figure 7 illustrates the wear intensity as a function of time. At first, the wear is significant, since the sur face defects are easily worn. The wear pattern (dδ/ds) only becomes steady after the first hour of testing. This is related to the high microhardness, low coefficient of friction of titanium nitride, and the plasticity of the sublayer of a solid solution of nitrogen in titanium. No stage of extreme wear was observed, since it requires a prolonged testing time and presupposes the destruc tion of the surface layer.
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based on CuSn acting as a solid lubricant enforced with ceramic inclusions of nanoWC, including Co and stellite walls, has the best coefficient of dry friction, which has been steady at a level of about 0.12. The application of a nanostructured ceramic phase makes it possible to obtain a finer microstructure of the composite material and, hence, more uniform wear. The wear mechanism varies from attrition at the initial stage to exfoliation at subsequent stages. The laser alloying of bearing steel with tin (Sn) makes it possible to obtain surface layers saturated with this element, which acts as a solid lubricating coating under sliding friction; they have low coeffi cients of friction and eliminate seizure and fatigue wear. The operation lifetime of an oil turbo drill after laser alloying with tin increased by 2.5 times compared with hardening by highfrequency currents. As a consequence of laser alloying from the gas phase (in nitrogen flow), we obtained a thin (less than 5 μm) surface layer of titanium nitride and a thick (150 μm) layer of nitrogen solid solution in titanium positioned beneath the thin layer. The nitrogen con centration in solid solution increased under the impact of the second and third laser pulse; optimum parameters of laser processing and surface plastic deformation were applied to obtain a surface layer that has minimum roughness without cracks and low resid ual stresses; the wear intensity of these processed disks was 2.3 × 10–5. Wear tests demonstrated the actual pos sibility of applying the developed method to the pro duction of frictional couples made of light titanium alloys. The work was carried out in the laboratory of inno vative additive technologies (Moscow State Techno logical University Stankin) in cooperation with the DIPI laboratory (École Nationale d’Ingénieurs de Saint Etienne, France) under the Decree of Govern 2014
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ment of Russian Federation No. 220 dated April 9, 2010 “On Measures Aimed at Involving Leading Scientists in Russian Educational Institutions” (additional agreement No. 1 to Contract No. 11.G34.31.0077 dated October 19, 2011).
8. 9.
NOTATION I—wear intensity; δ—linear wear; s—friction dis tance; qn—average normal loading per unit contact distance; θ1 and θ2—elastic constants of disk materi als; ρ—relative radius of curvature; Vs—sliding speed; V1,2—disk rolling speed; n1,2—disk rotation fre quency; t—test duration.
10.
11.
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