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Dry abrasion property of TiN-matrix coating deposited by reactive high velocity oxygen fuel (HVOF) spraying.

Abstract A TiN-matrix coating with a thickness of about 400 [micro]m was prepared by reactive high velocity oxygen fuel (HVOF) spraying on carbon steel in air. The phase composition, microhardness, and antiabrasion properties of the coating were investigated using x-ray diffraction (XRD), an energy dispersive spectrometer (EDS), a scanning electron microscope (SEM), a Vickers microhardness tester, and block-on-ring abrasive equipment. The abrasion mechanism of TiN-matrix coating under dry abrasion conditions was also discussed. The results indicate that the composition of the coating is main phases of TiN and [TiN.sub.0.3], minor phases of [Ti.sub.2] [O.sub.3], and [TiO.sub.2]. The TiN-matrix coating possesses high microhardness and relatively good toughness. Furthermore, the friction coefficient of the coating decreases with the increment of applied load. The interstratified distribution of titanium oxides can act as a solid lubricant during the wear test. The main abrasion mechanism of the TiN-matrix coating is adhesion wear. In addition, the coating with self-lubricating property can improve the antiwear property of the substrate significantly.

Keywords Reactive spraying, High velocity oxygen fuel (HVOF) spraying, Dry abrasion, Self-lubricating

Introduction

Recently, wear problems induced by abrasion have become a bottleneck of technological development in the industry that should be solved immediately. (1) Ceramic phase coatings--especially TiN films deposited on the surface of cutting tools for the protection of substrate--have been widely used in industry due to their high hardness, excellent wear resistance, and thermal stability. (2-4) At present, the main methods of preparation for TiN coatings are physical vapor deposition (PVD) (5) and chemical vapor deposition (CVD). (6) However, their further applications are restricted due to low deposition efficiency and thin coatings (below 10 [micro]m in thickness). A novel method of cold-spraying for depositing TiN-reinforced composite coatings had also been investigated because of its high deposition efficiency and relatively low deposition temperature, which can decrease the oxidation of coatings. (7), (8) However, the content of the TiN phase is relatively low (38.7 vol% (7)) due to its high melting point.

Reactive thermal spraying in an effort to obtain good TiN coating performance has attracted many researchers' interest due to the lack of pollution and excellent bonding strength of in situ technology for coatings. (9), (10) During spraying, hard ceramic phases (such as carbides, borides, and nitrides) are formed in situ by the reactions between the injected powder and the reactive gas. Using a gas tunnel-type plasma jet, Akira Kobayashi (11) prepared a TiN coating that was more than 200 [micro]m in thickness. Wenran Feng et al. (12) deposited a TiN coating that was 500 [micro]m in thickness using reactive plasma spraying in a nitrogen-containing environment. In addition, Lisong Xiao obtained nanostructured TiN coatings that were more than 300 [micro]m in thickness by means of reactive plasma spraying in air. (13)

However, many researchers (11-14) have just investigated the fundamental characteristics of TiN coating like hardness, coating thickness, phase composition, and so on. The abrasion resistance of TiN coating deposited by reactive thermal spraying, which is the most important performance in realistic applications, is rarely studied. Wenran Feng et al. (15) did some research on the tribological properties of TiN coatings prepared by reactive plasma spraying. In addition, TiN coatings prepared by reactive HVOF spraying were characterized by a considerable efficiency, significant hardness, and relatively good toughness. (16) The abrasion resistance of TiN coatings deposited by reactive HVOF spraying, however, has not yet been reported. In the present study, the abrasion resistance property of TiN-matrix coatings deposited by reactive HVOF spraying was investigated.

Experiments

Preparation of the sample and the coating

Medium carbon steel (0.42-0.50 wt% C steel), which was machined into standard samples (according to national standard of metallic materials wear test-GB 12444. 1-90 of China), was used as substrate material. Bearing steel of GCr15, which was machined into a ring (GB 12444.1-90), was used as a counterpart. Its hardness is about 62 HRC. (17) Commercial Ti powder, with a particle size from 45 to 70 [micro]m (Jin Jiang Metallic Powder Co. Ltd. of Shanghai, P.R. of China), was used as a starting material. Nitrogen was selected as carrier gas. After finely polished with 400 mesh abrasive paper, the cambered surface of substrates were grit-blasted with alumina to get a fresh surface, ultrasonically cleaned in anhydrous ethanol, and then dried in air prior to deposition. More than 300 [micro]m in thickness of TiN-matrix coating was deposited without bond coating by a JP5000 HVOF spraying instrument. The kerosene flow rate, oxygen flow rate, carrier gas flow rate ([N.sub.2]), and spray distance are 0.31 L/min, 822 L/min, 9.87 L/min, and 380 mm, respectively.

Wear tests

The surface of the specimen was ultrasonically cleaned in anhydrous ethanol to avoid the effect of surface pollution before wear tests, which used a block-on-ring sliding apparatus under dry conditions at room temperature. Figure 1 shows the schematic diagram of the block-on-ring equipment. The axle speed of the M-2000 wear test machine (Xuanhua Beilun Balancing Machinery Co. Ltd, China) was 400 r/min. The coated specimen was fixed using a sample holder, and the counterpart wear ring was placed under the sample. The applied load on the aforementioned coated specimen ranged from 350 N to 750 N. The average weight loss of the three specimens was taken at each load to decrease the error.

[FIGURE 1 OMITTED]

The counterpart wear ring was quenched to obtain the highest hardness of about 62 HRC. The abrasion behavior was recorded by an X-Y recorder during the wear test. The durative time of the wear test was 30 min. The weight loss of the coated specimens was measured using an electric analytic balance with 0.1 mg precision. The friction coefficient of the specimens was calculated using equation (1) (according to national standards of metallic materials wear test-GB 12444.1-90 of China),

[micro] = 1000 [M/R * F] * [[[alpha] + sin [alpha] * cos [alpha]]/2 sin [alpha]] (1)

[alpha] = arcsin [b/2r] (2)

where M, R, F, [alpha], and b in equations (1) and (2) are friction moments, the radius of the counterpart wear ring, the applied load, the contact angle, and the width of abrasion marks, respectively.

The surface and cross-section morphologies of the worn coated samples were studied with a scanning electron microscope (SEM, JSM-6460, Japan). An energy dispersive spectrometer (EDS, EDAX, USA) was used to investigate the chemical composition of the spallation debris from the coating. For comparison, an uncoated medium carbon steel specimen was tested as well.

Results and discussion

Phase composition and structure of the coating

From the XRD analysis (the version number of 2004 PDF card) shown in Fig. 2, it can be seen that the top coating is composed of TiN, [TiN.sub.0.3], [Ti.sub.2][O.sub.3], and [TiO.sub.2]. Five sharp TiN peaks appear in the spectrum, which suggests that the coating is mainly composed of the TiN phase. The contents of [TiN.sub.0.3] and [Ti.sub.2][O.sub.3] are moderate, whereas the intensity of [TiO.sub.2] is very low. That's the reason for naming the coating as a TiN-matrix coating. Figure 3 displays the SEM morphology of TiN-matrix coatings on the cross section. The coating has typical lamellar structure, with no distinct interfaces in the top coating and well bonding with the substrate--even without bond coat. The thickness of the coating is about 400 [micro]m, and the porosity of the coating is very low. From the consequence of EDS analysis shown in Fig. 4, it can be found that the bright phase (arrow 1) is composed of Ti, C, and N elements (Ti-50.45 at.%, C-22.57 at.%, N-26.98 at.%), while the dark phase (arrow 2) is composed of Ti, C, N, and O elements (Ti-30.01 at.%, C-19.82 at.%, N-17.79 at.%, O-32.38 at.%). To enhance the conductibility of TiN-matrix coatings, element C was sputtered on the cross section of the coating. Combined with an XRD spectrum it is found that the bright phase consists of titanium nitrides (TiN and [TiN.sub.0.3]), and the other consists of the mixture phase of titanium nitrides (TiN and [TiN.sub.0.3]) and titanium oxides ([Ti.sub.2][O.sub.3] and [TiO.sub.2]). Compared with the TiN coating that was prepared using reactive plasma spraying (RPS), (13) [Ti.sub.2][O.sub.3] and [TiO.sub.2] phases were formed due to the oxygen used for combustion and deposition in the atmosphere during spraying.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Vickers microhardness and indentation

The average Vickers microhardness value of TiN-matrix coatings on the cross section is 1137 [+ or -] 172 HV at 0.05 kg applied load, which is much higher than the TiN coatings prepared by RPS at the same applied load in earlier research (13) due to the high kinetic energy of powder, adequate deformation, and dense coatings supplied by HVOF spraying. The indentation morphology on the cross section of TiN-matrix coatings is displayed in Fig. 5. The brim and diagonal of indentation are very clear, and little microcracking is found around the indentation. It is shown that the TiN-matrix coating not only has high microhardness, but also possesses relatively good toughness. The characteristics of HVOF spraying and the titanium oxides formed in situ may be responsible for this phenomenon. First of all, HVOF spraying with a high velocity of combustion gases makes the coating dense, and the phases generated in situ produce no pollution. They can hinder the formation and propagation of the micro-crack. Secondly, although the existence of titanium oxides decreases the microhardness of the coating to some extent due to their lower microhardness, their interstratified structure in the main TiN phase may be an important factor in slowing down the velocity of crack initiation.

[FIGURE 5 OMITTED]

Weight loss and friction coefficient of TiN-matrix coating

The relationship between weight loss and the variation of applied load is shown in Fig. 6. It can be seen that weight loss increases with the increment of applied load. However, the weight loss increases a little when the applied load exceeds 300 N. The weight loss of the substrate (medium carbon steel) without a TiN-matrix coating is about 1,500 times that of the sample with a TiN-matrix coating at 100 N applied load. Figure 7 displays the relationship between the friction coefficient and applied load. It is contrary to the relationship between the weight loss and applied load. The friction coefficient decreases with the increment of applied load; however, the friction coefficient decreases a little when the applied load exceeds 300 N. The variable trend of the friction coefficient is similar to the research investigated in the literature. (15)

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

From these two diagrams, it can be concluded that the TiN-matrix coating possesses excellent abrasion resistance and self-lubricating properties, especially under high applied load. It also shows that the friction coefficient may arrive at a stable value, which cannot decrease with the increment of applied load. Several reasons may contribute to this phenomenon. First, the surface temperature of the coating will increase a lot under the sliding friction of samples, especially using high applied load under dry abrasion conditions. The oxidation film formed on the surface of a TiN-matrix coating and the counterpart wear ring can decrease the weight loss and friction coefficient to some extent. However, the friction coefficient will not decrease significantly with a formed oxidation film. The formation of an oxidation film needs definite temperature that increases with the increment of applied load. The applied load has little influence when the oxidation film is formed. Secondly, the interstratified distribution of titanium oxides can act as solid lubricant during the wear test. (18) It is very useful to decrease the friction coefficient and the weight loss of the coating.

Wear mechanism

The SEM morphologies of the surface wear track for the counterpart ring, the sample without a coating, the sample with a TiN-matrix coating at 300 N applied load, and a TiN-matrix coating at 750 N applied load are shown in Fig. 8a-d, respectively. It is evident that the surface of the sample without a coating (Fig. 8a) displays a wave shape of plastic deformation parallel to the sliding direction. It can occur under the condition of lower stiffness but higher toughness of the substrate material than abrasion counterpart ring. (19) The deep plough can be seen from the surface of the counterpart ring, and the surfaces of TiN-matrix coatings under different applied loads are relatively smooth without obvious plough due to their higher microhardness (Fig. 8b-d). The TiN-matrix coating with high stiffness ploughs and cuts the surface of the counterpart ring during the wear test. We can also conclude that the TiN-matrix coating possesses a self-lubricating property due to a more smooth surface from Figs. 8c and 8d.

[FIGURE 8 OMITTED]

The TiN and titanium oxide debris was found on the surface of the counterpart wear ring after lengthy dry abrasion according to EDS analysis in Fig. 9a. The selected composition of the debris on the surface of the counterpart ring is 9.11 at.% N element, 68.78 at.% O element, 18.89 at.% Ti element, and 3.23 at.% Fe element. Reversely, the composition of the TiN-matrix coating surface is 5.30 at.% N element, 63.50 at.% O element, 14.49 at.% Ti element, 16.28 at.% Fe element, and a little chromium (Cr) element (0.43 at.%) in Fig. 9b. It can be concluded that the titanium oxides, titanium nitrides, and iron oxides phases appear on the abrasion surface. The titanium oxides and titanium nitrides phases transferred from the coating to the wear ring by adhesion explain that the wear mechanism of the TiN-matrix coating is the main adhesion wear. The iron oxides and chromium elements from the ring arc also detected on the surface of TiN-matrix coatings (Fig. 9b). The reason is that the higher microhardness of TiN-matrix coatings than the ring ploughs the ring to form debris, and some of them remain on the surface of the coating. In addition, the TiN-matrix coating will flake off, partially due to fatigue failure induced by stress after lengthy dry abrasion. The debris is ground and flaked off from the ring and the coating after lengthy dry abrasion. The crushed debris can act as an abrasive to decrease the weight loss during the continuous wear test.

[FIGURE 9 OMITTED]

The cross-section images of TiN-matrix coatings after dry abrasion tests at 300 N and 750 N applied load are shown in Fig. 10. Both of the TiN-matrix coatings are spalling from the surface of the coatings. However, parallel and vertical cracks generate and propagate in the subsurface of the coating with heavier applied load. The stress concentration and the microcrack propagation should be responsible for the spalling of TiN-matrix coatings. It is well-known that there is some residual stress in the coatings due to rapid cooling during the preparation of the coating. Tangential stress and deformation will generate on the surface of coatings during the wear test. During deformation, the stress concentrates in the origin of the microcrack and the microcrack propagates to relieve the stress. When the microcrack penetrates from the subsurface to the surface, the TiN-matrix coating debris will flake off the coating, leading to fatigue failure of the coating. Heavier applied load will generate more serious stress concentration and microcrack propagation.

[FIGURE 10 OMITTED]

Conclusions

Thick TiN-matrix coatings with high microhardness and relatively good toughness was prepared successfully by reactive HVOF spraying in air. The thickness of the coating was about 400 [micro]m and the porosity of the coating was very low. The composition of the coating was main phase TiN and [TiN.sub.0.3], and minor phase [Ti.sub.2][O.sub.3] and [TiO.sub.2].

The TiN-matrix coating possesses self-lubricating properties. The amplitude of weight loss and the friction coefficient decrease with the increment of applied load. The interstratified distribution of titanium oxides, and the oxidation film formed during the wear test, can decrease the friction coefficient of the coating to some extent. But the applied load has little influence when the oxidation film is formed. The main abrasive mechanism of the TiN-matrix coating is adhesion wear.

Acknowledgments This work was financially supported by the Opening Fund Program of China (No. KFJJ07-2). The authors are grateful to Mr. W. Tang for depositing the coating. The authors are also thankful for the hard work of the editors and reviewers on the paper.

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[C] FSCT and OCCA 2009

Z. Mao, J. Ma, J. Wang, B. Sun ([??])

State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, China e-mail: bdsun@sjtu.edu.cn

J. Ma

Material Institute, Hebei University of Science and Technology, Shijiazhuang 050054, China

Z. Mao

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China e-mail: mzp@sjtu.edu.cn
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Author:Mao, Zhengping; Ma, Jing; Wang, Jun; Sun, Baode
Publication:JCT Research
Date:Mar 1, 2010
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