Abstract
Two kinds of TiC reinforced wear resistant coatings with or without the addition of Mo were prepared by plasma spraying. Ultrafine TiC powders with sizes varying from 5 to 15 μm were introduced as the reinforcement phase and NiCr alloy as the binder phase. Phase composition and microstructure of the as prepared coatings were characterised by X-ray diffraction and scanning electron microscopy. Wear behaviour of the TiC reinforced coatings was also investigated using a ring-on-block tester. Results showed that the TiC reinforced coatings adhered well to the substrate and exhibited dense structure with low porosity, even though oxidation occurred during spraying. Wear tests indicated that the sliding wear performance of the NiCrMo–TiC coating was better than that of the NiCr–TiC coating at different forces under the same experimental conditions.
Introduction
Plasma spraying is a widely well established method to produce wear and corrosion resistant coatings. Among various types of coatings, WC-based1 – 5 and Cr3C2-based6,7 coatings have received comprehensive attention due to their excellent wear resistance properties. It is generally accepted that WC–Co coatings possess excellent antiwear performance at low temperatures,1 – 3 but the formation of brittle phases, including CoxWyCz and W2C and other complex amorphous phases, resulting from the decomposition of WC, could undoubtedly hinder their further application.2,3 As in the case of NiCr–Cr3C2 coatings, although the anticorrosion property is intriguing, the wear resistance is about a magnitude lower in comparison with WC–Co coatings.6 With high hardness, attractive wear, low density, excellent corrosion resistance and favourable thermal and thermodynamic stability, TiC reinforced coatings have attracted tremendous research interests in the past few years.8 Up to now, a significant fraction of work has been performed on the fabrication of TiC reinforced coatings along together with their tribological behaviours. For example, NiCrBSi–TiC coating fabricated by atmospheric and vacuum plasma spraying has been systematically investigated long time ago and their wear behaviours under various loads and sliding speeds have simultaneously been evaluated.9 In addition, NiCr–(Ti,Ta)C/TaC coatings,10 TiC–Fe coatings11 – 13 and TiC–Ti coatings8 have also been thoroughly studied. Recently, nanocrystalline coatings of (Ti,Mo)(C,N)–45 vol.-%(Ni–20 wt-%Co) were produced by a combination of high energy milling and thermal spraying, which exhibited improved overall performance compared to traditional coatings.14
It is well known that properties especially the wear resistance of carbide reinforced coatings could be enhanced by improving the quality of the spraying feedstock, e.g. reducing the size of the reinforcing phase. In comparison to various investigations on preparation and tribological properties of TiC based coatings, there have been few research related to this. In this work, in order to make the best use of the excellent properties of TiC, ultrafine TiC powders with sizes varying from 5 to 15 μm were introduced. NiCr solid solution instead of single Ni was selected as matrix, and Mo was added to improve the wettability between ceramic phase TiC and metallic phase NiCr. In fact, Mo has been commonly used as addition in cermets in order to improve the wetting of carbide phase by the metal matrix and enhance the fracture toughness due to its high self-adhesion, high hardness and good wear resistance. Additionally, thermal sprayed Mo based coatings have also found widely application in friction fields.10,15 Two kinds of coatings, namely, NiCr–TiC coatings and NiCrMo–TiC coatings, were deposited onto A3 steel by plasma spraying. Phase composition, microstructure and tribological properties of the as prepared coatings were investigated. The effect of Mo on the structure and wear resistance of TiC reinforced coatings was systematically evaluated and possible mechanism was also discussed.
Experimental
The powders used in this work are supplied by Precursor (Yiyang) Plasma Powders Co., Ltd. The morphologies of particles used for the processing of spraying powder are shown in Fig. 1a–c. It is apparent that the morphology of TiC powders is irregular with diameter varying from 5 to 15 μm. Sintered and crushed 80Ni–20Cr (wt.-%) alloy powder exhibits angular morphology with a size distribution of 45–75 μm, and the agglomerated and sintered Mo powders are spherical with a size distribution of 38–55 μm. NiCr alloy powder is the starting metal matrix, TiC and Mo powders were added to the matrix at 40 and 20 vol.-% respectively, or just TiC powders added at 40 vol.-%. It has been comprehensively recognised that coatings fabricated with agglomerated and sintered powders show better performance than that only with agglomerated or mechanical mixed powders. Therefore, agglomerated and sintered powders were accepted in our research. In order to break physical agglomeration, ultrafine TiC particles were firstly ultrasonic treated in distilled water along with magnetic stirring. Polyvinyl alcohol aqueous solution (5 wt.-%) and microsized NiCr and Mo powders were added to make a sol solution. After stirring for ∼2 h, the sol solution was located in furnace at 60°C for ∼4 h to remove remnant water and then became a compact. The compact was then cold isostatic pressed under pressure of 2×105 N, which was subsequently sintered in a vacuum furnace at 1600°C for 2 h. Finally, the sintered compact was crushed to spraying feedstock. As shown in Fig. 1d, the agglomerated and sintered powder is a mixture of near spherical particles and irregular particles. After seizing, the final spraying feedstock had a size distribution of −45 to +15 μm, which was suitable for spraying. Before spraying, the spraying feedstock was dried in a furnace at 120°C for 24 h in order to remove excess moisture. The substrate, i.e. A3 steel, was rinsed by acetone and then grit blasted with 40 mesh alumina particles.
Images (SEM) of starting powders
A plasma praying gun (supplied by Praxair Ltd, USA) equipped with an internal powder injection was introduced. The spraying gun is fixed onto a two axial manipulator which is connected to a computer to control the transverse velocity and the vertical pace of the spraying gun when spraying planar sample. Argon gas was selected as the primary gas and helium gas as the secondary gas by virtue of its high enthalpy. All the gases are with high purity of 99·99%. For the purpose of reducing thermal stress induced by the great thermal gradient between the as produced coating and substrate, before spraying process, the substrate was subject to two passes of the plasma plume without injecting powders to be preheated. Coatings were fabricated by manifold cycles of spraying and compressed air gas was used to cool the substrate aiming at decreasing the negative impact of the great thermal gradient during spraying. The thickness of each spraying cycle and final coatings was about 50 and 200 μm respectively. The processing parameters are shown in Table 1.
Plasma spraying conditions of coatings
An X-ray diffractometer (JE0L Rigaku 2500/PC) was used to analyse the phase composition of the plasma sprayed coatings at a scanning rate of 0·02 s−1 and time step of 2 s, 2θ ranging from 30 to 90°. The starting powders and the as sprayed coatings were structurally characterised by SEM (JEOL JSM-5600) utilising backscattered imaging mode equipped with an energy dispersive spectroscope. Microhardness measurements were conducted on polished cross-sections of the coatings using a Vickers indenter and a load of 100 g (HV0·1). The values presented are averages of 10 readings.
Wear resistance tests were performed using a block-on-ring tester (M-2000; ChengXing Testing Equipment Manufacture Co. Ltd, Zhangjiakou, China) with quenched and tempered W–18Cr–4V steel used as counterpart rotating at 400 rev min−1 under dry sliding condition. The schematic illustration of the block-on-ring wear test is shown in Fig. 2. The block specimens were obtained by wire electrode cutting, and their lengths and widths are 20 and 3 mm respectively. The lengths of the wear scars are identical to the width of the block, i.e. 3 mm, while the widths of the wear scars were measured by stereomicroscope every 5 min during wear tests, as shown in Fig. 2. Two loads, 50 and 100 N, were applied for the wear test. The wear loss volume of the coatings could be calculated by equation (1)
Schematic illustration of block-on-ring wear test
Results and discussion
X-ray diffraction analysis
X-ray diffraction spectra of the spraying powders and the as prepared coatings are shown in Fig. 3. In the case of NiCr–TiC coatings, two distinct phases, NiCr phase and TiC phase, could be clearly distinguished. Mo phase can also be indexed in the NiCrMo–TiC coating, which results from the addition of Mo powder in the original composite powder. It is noteworthy that TiO2 phase could be undoubtedly assigned in both coatings irrespective of the addition of Mo or not, giving evidence that TiC powders were oxidised during spraying. Although the inert gases and the supersonic flame velocity can protect TiC from oxidising at a certain extent, the ultrafine TiC powder with high surface activity can absorb energy much easier, leading to excessive oxidation. Comparing the relative intensity of diffraction peaks of the powders and coatings, it should be noted that the amount of TiC underwent pronounced loss, and this is more severe for the NiCrMo–TiC coating. Several reasons can be taken into account for this phenomenon: first of all, as has been discussed above, ultrafine TiC powders were oxidised during spraying in both coatings; second, a small amount of TiC dissolved in molten binder phase but did not reprecipitate on cooling. In addition, as TiC exhibits a high melting point and low density, a small amount of totally unmelted TiC particles can be easily bounced back or blown away when impinging against the substrate at high velocity. For the NiCrMo–TiC coating prepared by plasma spraying, MoO2 and Mo2C phases have been previously detected.10,16 In this work, Mo2C phase can be also identified, as shown in Fig. 3b, but MoO2 is difficult to be assigned. The appearance of Mo2C may be attributed to the following reactions
X-ray diffraction spectra of spraying powders and as prepared coatings
Coating microstructure
The cross-section structures of the TiC reinforced coatings are shown in Fig. 4. It is clear that both coatings are dense with typical morphology characteristics of plasma spraying coatings. There is no evidence of delamination in both coatings, indicating the excellent bond between reinforcing and binder phases. From the results of surface scanning, we are firmly convinced that the dark grey part is TiC reinforce phase, the light grey is NiCr solid solution and the dark grey part at splats boundaries and inside TiC particles is oxide, which cannot be easily discriminated by SEM images (Fig. 4); the white part in the NiCrMo–TiC coating is Mo. Some pores (dark part), typically larger than reinforce phase particles, are also visible in both coatings. In the NiCr–TiC coating, TiC particles seem to be more homogenously distributed than in the NiCrMo–TiC coating. Some TiC particles embedded in the binder phase (e.g. NiCr) in the NiCr–TiC coating exhibit a spherical shape similar to that of the original spraying powders, indicating that they were just partially melted on the surface during spraying. It can also be observed that, after spraying, the size of some TiC particles increased to ∼20 μm in both coatings. The emergence of larger TiC particles may result from the coalescence of small TiC particles. Because of the high surface activity, the ultrafine particles can be easily agglomerated during the preparation of feedstock for spraying, and then the partially or fully melted small particles adhered together during spraying and finally formed the larger particles. Based on the backscattered image analysis methods of thermal sprayed coating, it can be roughly obtained that TiC content reduced from 40 vol.-% in the original spraying powders to less than 30 vol.-% in both coatings, which agrees well with the decreased relative diffraction intensity of TiC after spraying (Fig. 3).18 The main reason could be due to the oxidation of the ultrafine TiC particles. The dissociation of TiC and reaction between Mo and TiC should also be considered for the reduction in TiC content. Additionally, as has been discussed above, a small amount of totally unmelted TiC particles can be easily bounced back or blown away, further increasing the TiC reduction. On the contrary, the Mo content remained unchanged after spraying, equivalent to 20 vol.-%. Furthermore, the NiCrMo–TiC coating exhibited higher porosity (∼2·99%) than that of the NiCr–TiC coating (∼2·3%), and the porosity in NiCrMo–TiC mainly existed near the interfaces between NiCr and Mo phases, as shown in Fig. 4. For interpretation of the higher porosity in the NiCrMo–TiC coating, the mismatching of thermal expansion coefficient of different phases, which lead to the inconsistent solidification rate, need to be considered first. In addition, the high thermal gradient of the plasma flame should also be taken into account. Despite of the high temperature (exceed 30 000 K) in the centreline of plasma flame, the temperature at periphery of the flame is much lower (2000 K) due to the thermal gradient. Therefore, a small amount of Mo and TiC powders around the periphery of plasma flame was just partially melted or even fully unmelted, and these partially melted or unmelted powders were only overlapped onto substrate or the presprayed splats by plastic deformation, giving rise to more porosity. On the contrary, as for the NiCr–TiC coating, NiCr alloy powders can be thoroughly melted and then encase the partially melted or unmelted TiC powders to prevent the formation of porosity.
Backscattered images of cross-section of a NiCr–TiC coating and b NiCrMo–TiC coating
As circle marked in Fig. 4, unique cladding structures formed in both coatings. In the case of TiC–NiCr coatings, the presence of Ti–Ni–Cr carbide dendrite phase has been previously reported,19 which can contribute to the formation of a cladding structure between reinforcing and binder phases (Fig. 4a). Meanwhile, (Ti,Mo)C solid solution was universally observed in TiC based cermets or cutting tools with the addition of Mo,13 via element diffusion. Therefore, it is reasonable to propose that Mo element diffused towards the surface of TiC crystalline during spraying and displaced some Ti atoms. As a result, (Ti,Mo)C solid solution surrounding TiC was formed. Because the crystal structure of (Ti,Mo)C is almost the same as that of TiC, it is quite difficult to be detected by X-ray traces.17 Furthermore, Mo generally existed in an elemental form and then formed Ni–Cr–Mo and bcc Mo–Cr rich phases in coatings with NiCr being the binder phase, which can also lead to the formation of the cladding structure between Mo phase and NiCr phase. The cladding structure can serve as the interface between different phases to improve the cohesion strength of coatings and accordingly increase their wear resistance.
Coating hardness
Based on the hardness tests, the average Vickers hardness of NiCr–TiC and NiCrMo–TiC coatings are 600 and 660 HV0·1. As a matter of fact, the hardness of coatings varied between different areas. A high value of hardness, i.e. up to 1400 HV0·1, can be measured in TiC rich zone, but the value drops to about 400 HV0·1 in the binder phase. The final values presented are averages of 10 readings along the direction parallel to the interface.
Wear test analysis
The wear loss volumes of the as sprayed coatings using applied forces of 50 and 100 N after 20 min were elucidated in Fig. 5. As shown in the histogram, NiCrMo–TiC coating shows superior wear resistance to the NiCr–TiC coating at different forces with the same technological parameters during tests. When applied load was increased from 50 to 100 N, the wear loss volume of the NiCrMo–TiC coating increased by ∼19%, slightly lower than that of the NiCr–TiC coating which was ∼21%. As has been previously demonstrated, modification of both reinforce phase and matrix contributes greatly to the improvement of mechanical properties of carbide reinforced cermets.20,21 Many researches have indicated that molybdenum as an alloying element can strongly improve the wettability of TiC reinforce phase by Ni based binder,22 and subsequently enhance the wear resistance. In this study, evident improvement of wear resistance also happened with the addition of Mo. Two major features should be considered. As discussed above, the microhardness of the NiCr–TiC coating is slightly lower than that of the NiCrMo–TiC coating, which can inevitably deteriorate its wear resistance. On the other hand, cohesion strength between reinforcing and binder phases also plays a crucial role in determining the wear resistance of coatings. The formation of Mo2C in the NiCrMo–TiC coating can effectively enhance the adhesion strength of the carbide/matrix interface. During sliding against the counterpart, the strong carbide/matrix interface will prohibit the pull-out of particles, finally resulting in a lower wear volume. The superior wear resistance of the NiCrMo–TiC coating can further be verified by its low friction coefficients, which was ∼0·65 under the load of 50 N. By comparison, the friction coefficients of the substrate and TiC–NiCr coating were about 0·92 and 0·7 respectively.
Wear volume losses of TiC reinforced coatings after 20 min under load of 50 and 100 N
For both coatings, when lower load was applied, the reinforcement TiC particles adhere well to NiCr matrix, the hard TiC particles can bear the applied load and hinder the processing of abrasion. When the applied force was elevated to 100 N, some TiC particles with large sizes can be pulled out and made the binder phase exposing to the abrasive counterbody. The pulled-out hard particles may probably serve as a third body between friction interfaces and then promote the wear rates of coatings. Therefore, the volume losses of both coatings increased with elevated load, as shown in Fig. 5.
Conclusions
In this paper, two kinds of TiC reinforced coatings with or without the addition of Mo were successfully fabricated via plasma spraying by using agglomerated and sintered spraying powders. Although small amount of TiO2 can be detected due to the oxidation of TiC particles, both coatings were dense with low porosity (<3%). With the addition of Mo, Mo2C phase were detected in the NiCrMo–TiC coating. Cladding structure between different phases was formed in both coatings. The wear test results showed that of the NiCrMo–-TiC coating exhibited superior wear resistance to the NiCr–TiC coating at different forces under the same experimental conditions, and the wear volume losses increased with elevated force for both coatings.