Sliding wear performance of nickel-based cermet coatings composed of WC and Al2O3 nanosized particles

Surface & Coatings Technology 304 (2016) 401–412

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Sliding wear performance of nickel-based cermet coatings composed of WC and Al2O3 nanosized particles M.A. Farrokhzad a,b,⁎, T.I. Khan b,c a b c

Alberta Innovates Future Technologies R&D Research Associate, Canada The University of Calgary, 2500 University Drive NW, Calgary, Canada University of Bradford, School of Engineering, Bradford, West Yorkshire, United Kingdom

a r t i c l e

i n f o

Article history: Received 16 March 2016 Revised 29 June 2016 Accepted in revised form 4 July 2016 Available online 7 July 2016 Keywords: Co-electrodeposition Ceramic-metallic coatings Cermet Sliding wear test

a b s t r a c t This paper investigates the sliding wear performance of two types of co-electrodeposited cermet coatings composed of nano-sized tungsten carbide (WC) and combined tungsten carbide and alumina (Al2O3) particles incorporated in a nickel matrix. For this purpose, the effects of alternating the ceramic particle concentration in the electrolyte solutions on microhardness of the coatings and also the effect of applied loads on wear performance of the coatings have been studied using ball-on-flat sliding wear tests. The wear track volumes and the progression of wear depths as a function of time and at three applied loads were recorded and wear track morphologies were investigated using FE-SEM and microhardness testing. The results showed that microstructure, microhardness and wear performance of the coatings composed of WC improved when Al2O3 particles were introduced into the matrix. It was also found that the rule of mixtures for composite materials provides a good explanation for microhardness behaviour while Archard equation can explain the changes in wear performance due to the hardness and microstructural changes. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Co-electrodeposited ceramic-metallic (cermet) materials are a group of coating materials in which the mechanical properties of a metallic matrix are enhanced with the addition of nanosized ultra-hard ceramic particles (such as metallic carbides or oxides) [1,2]. Direct current co-electrodeposition is one of the most widely used techniques for producing cermet composites materials [3]. The most common application of these materials can be found in industries where wear or erosion processes are detrimental to the surface quality of steel and alloy components. To improve the wear resistance of steel and alloyed components against moving hard particles, applying cermet coatings on their substrate (with an adequate thickness) can improve the life of the components significantly. Many types of nanosized ceramic particles can be incorporated in a nickel matrix by the co-electrodeposition process. Among them, alumina (Al2O3) and tungsten carbide (WC) are widely used to improve the hardness and wear resistance of the cermet coating [4–7]. The approximate average hardness for alumina is 2300 HV and 2000 HV for tungsten carbide. Therefore, based on the rule of mixture for composite materials, when sufficient amount of ceramic particles

⁎ Corresponding author at: Alberta Innovates Future Technologies R&D Research Associate, Canada. E-mail address: [email protected] (M.A. Farrokhzad).

http://dx.doi.org/10.1016/j.surfcoat.2016.07.014 0257-8972/© 2016 Elsevier B.V. All rights reserved.

is incorporated in the nickel matrix, an improvement in the coating hardness can be expected. At optimized conditions, the microhardness of nickel-based cermet coatings containing Al2O3 nanosized particles can reach 700 HV. Similarly nickel-based cermet coatings containing WC nanosized particles can have a Vickers microhardness between 300 and 500 HV [1,7,8]. The hardness values of composite cermet coatings depend not only on the quantity of the incorporated ceramic particles in the matrix but also on particle size and uniformity of particle distribution within the metallic matrix. It is generally agreed that the improved microhardness of cermet coatings is due to the combined effects of grain refinement within the matrix by Hall-Petch mechanism and also dispersion strengthening by Orowan mechanism [6,9]. The volume percent of particles incorporated in the matrix is influenced by several parameters such as, the composition of the electrolyte, the type of particles and their concentration in the electrolyte, use of surfactants, hydrodynamics around the electrodes and applied current density [10–13]. A higher volume of incorporated ceramic particles and their uniform distribution throughout the metallic matrix improve the tribological and the wear resistance properties of the cermet coatings. There are many testing techniques that can be employed to study the wear performance of coated materials. Most of these techniques focus on characterizing the wear performance by measuring either the wear volume, wear depth or wear coefficient. The main focuses of these testing factors are on test procedure, material characteristics, applied load, surface conditions, environment and test duration. Kennedy et al. have provided a list of factors that if followed, can help choose a suitable wear

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Table 1 Electrolyte bath composition and ceramic particle concentrations of electrolyte solutions. Coating

Particle load (molarity and mass) in electrolyte

Electrolyte composition

Ni + WC (0.25 mol) Ni + WC (0.50 mol) Ni + WC/Al2O3 (0.25 mol) Ni + WC/Al2O3 (0.50 mol)

WC: 0.25 mol (49.713 g/lit) WC: 0.50 mol (99.425 g/lit) WC: 0.125 mol (24.856 g/lit) + Al2O3: 0.125 mol (12.745 g/lit) WC: 0.25 mol (49.713 g/lit) + Al2O3: 0.25 mol (25.490 g/lit)

1 M of nickel sulphate hexahydrate (NiSO4·6H2O); 0.2 M of nickel chloride hexahydrate (NiCl2·6H2O) 0.5 M of boric acid, 0.5 g/lit saccharin Distilled water

test for a selected coating material. Special attention to detail for each factor can ensure accuracy, precision and a higher level of repeatability for the wear tests [14]. Although measuring the weight loss caused by wear is a common practice for measuring wear performance, it is not always the most accurate method for thin coatings. For thin coatings, a calculation of the wear volume (or wear depth) was found to be a more practical approach when the geometry of the wear crater is regular, symmetrical and uniform. In this condition, the wear volume can be calculated by measuring the depth, width and length of the wear track [14]. Variability in measurement of the wear volumes are expected for wear tests conducted in dry condition. Blau et al. have reported a coefficient of variation range from 2.4% to 37% for the within laboratory data in dry conditions. Reportedly, reciprocating wear test (based on ASTM G133) have the highest level of variation among all other methods [15]. The variations in wear volume measurements can come from three main sources; the machine (both construction and calibration sources), operating/operator technique and, finally mechanical properties and the microstructure nature of tested materials [15]. Another significant source of error for wear testing can be the precision level from the instruments used to measure the wear track dimensions (i.e. profilometer for the depth). Therefore, it is essential that useful statistical techniques be used to identify the confidence intervals for the calculated wear characteristics such as the wear depth or volume. For this paper, the direct current co-electrodeposition method was used to develop two groups of nickel-based cermet coatings composed of; only nanosized tungsten carbide (WC) particles (group-1) and mixtures of nanosized tungsten carbide (WC) with the addition of alumina (Al2O3) particles (group-2). The wear performance of nickel-based cermet coating composed of only WC or Al2O3 has been studied in the past (cited in this section) but to the best of the authors' knowledge, there has been no study on wear performance of the nickel-based cermet composed of both WC and Al2O3 particles in a nickel matrix. In addition, most of the literature that can be found on wear of cermet coatings composed of only one type of particles (either WC or Al2O3) have used arbitrary test parameters for the wear tests and did not fully follow the guidelines provided by ASTM G133 (Standard Test Method for Linear Reciprocation Ball-on-Flat Sliding Wear) to any extent. Thus, another goal of this research has become to generate comparable sliding wear

test results of the above-mentioned coatings following several guidelines provided in ASTM G133. Based on the obtained wear results, we have explored the potential correlation between coating characteristics (i.e. particle volume percent) to their microhardness and wear performance. The Vickers microhardness test was used for measuring the microhardness of the cross-sections of the coatings. Sliding wear tests were conducted and the volumes for wear tracks have been calculated using the formulations described in ASTM G133. Additional estimates of wear rates (wear depth progression as a function of time) have been calculated based on the recorded responses from a linear variable differential transformer (LVDT) installed on the wear test apparatus for each applied load and coating type. Depth and cross-section profiles of the wear tracks were measured using a profilometer. The wear tracks were investigated using field emission scanning electron microscopy (FE-SEM). The microhardness of wear tracks were also measured and compared to the unworn microhardness values for the coatings. Finally, the potential wear mechanisms during the course of sliding have been analyzed and discussed. 2. Experimental procedure 2.1. Materials Two types of powders were co-electrodeposited with nickel for this study; tungsten carbide (WC) purchased from US Research Nanomaterials, Inc. (USA) and alumina (α-Al2O3) purchased from M K Impex Corp. Ltd. The purity for WC was 99% and 99.95% for α-Al2O3. The as-received average grain size for WC was 55 nm and 20 nm for Al2O3. The anode was cut from a nickel rod (99.9% purity). The substrate material was made from hot-rolled AISI-1018 carbon steel bars and specimens for co-electrodeposition were cut in a rectangular shape (length: 25.4 mm, width: 12.7 mm and thickness: 6.5 mm) using a water-jet. The mill scale was removed by sand-blasting and the specimens then grinded with a 600 grit sand-paper. The substrate specimens were then submerged in two alkaline solutions (E-Kleen 102-E™ and EKleen 129-L™) to remove any residual grease. Prior to submerging the specimens in the electrolyte for the co-electrodeposition process, they were also placed in an acidic solution (acid pickling solution with 20%

Fig. 1. Transmission electron microscopy (TEM) of the WC and Al2O3 particles.

M.A. Farrokhzad, T.I. Khan / Surface & Coatings Technology 304 (2016) 401–412 Table 2 Particle content (wt%) in the coatings.

Coating

WC

Al2O3

Total particle content in Ni matrix

Ni + WC (0.25 mol) Ni + WC (0.50 mol) Ni + WC/Al2O3 (0.25 mol) Ni + WC/Al2O3 (0.50 mol)

9.5% ± 1.5% 12.5% ± 1.5% 5.5% ± 1.5% 6.5% ± 1.5%

n.a. n.a. 9.5% ± 1.0% 10.5% ± 1.0%

9.5% ± 1.5% 12.5% ± 1.5% 15.0% ± 2.5% 17.0% ± 2.5%

HCl and 80% distilled water) to remove any remaining contaminates or oxides and to activate the surface for co-electrodeposition. 2.2. Co-electrodeposition The co-electrodeposition process was based on applying direct current to the electrodes (2 A/dm2) for 4 h. The produced thickness was measured to be 160 ± 30 μm. The formula for electrolyte solution was based on Watt's bath solutions. The concentrations of powders in the electrolyte are given in Table 1 and were originated from previous research [3,7,16]. A very small amount of Saccharin was added to the electrolyte solutions for grain refinement of the nickel matrix [17]. The pH of the electrolyte solutions was measured to be 4.0 to 4.5. The electrolyte solution was heated to 50 °C and kept at this temperature during the co-electrodeposition process [4]. The electrolyte solutions were constantly stirred by a magnetic stirrer to suspend the particles and prevent agglomeration in the solutions during the co-electrodeposition process. The stirring velocity was kept at 350 to 380 rpm. The cathode current efficiency (CCE) was measured at 80% to 85%. 2.3. Vickers microhardness The cross-sections of the coatings for the microhardness testing were prepared by polishing to 0.25 μm using diamond pastes. Based on the procedure provided by ASTM E384-10, the microhardness tests were carried out across the transverse sections of the coating using a Leitz Miniload-2 microhardness Vickers indenter (equipped with a diamond pyramid indenter) operating under a 25 g load. Fifteen indentations were recorded for each coating at the center of the coating thickness. The following equation was used to calculate the average microhardness for the coating: HV ¼ 1854:4

P d

2

ð1Þ

where P is gram-force (gf) and d is the average of the two diagonals of the recorded indentation in micrometers.

403

wear performance of the coatings. The test parameters were chosen based on a previous study on similar materials [16]. The balls used for sliding were made of alumina (Al2O3) with an average Vickers hardness of 2600 to 2720 kg/mm/mm [18]. The balls slide reciprocally against the coated specimens. Wear depths as a function of time were recorded. The test was repeated 4 times for each type of coating and under each load to ensure reproducibility of results. A 16-bit 100 kHz data acquisition system (USB-1608FS model from micro DAQ, OH, USA) was used to collect data. The wear tracks were cleaned with acetone prior to depth measurements. Three readings of the wear depths of wear tracks were measured (at the center and at each side) using a Mitutoyo profilometer model SJ-210. The stylus radius of the profilometer detector was 2 μm (at a 60° angle) and the measuring force was 0.75 mN. The lengths and widths of wear tracks were measured three times each to 0.1 mm precision using optical microscope equipped with a measuring ruler slide. The morphology of wear tracks was studied using FE-SEM. 2.4.1. Alternations to G-133 testing procedure The tests results provided here are not in full compliance with the provisions of Test Method G133, Procedure A, because the test time duration was selected to be 30 min (1800 s) instead of 16 min and 40 s (and for an oscillating frequency of 1.39 Hz instead of 5 Hz) as prescribed by the standard. The changes in time and oscillating frequency meant that the material was subjected to sliding wear distance of 50 m instead of 100 m as prescribed by the standard. Also, two additional forces (15.0 N and 60.0 N) were included in the tests along with the required 25.0 N normal load by the standard. Due to the local climate and air conditions of the lab, the relative humidity was monitored to be 25% ± 2.5% which differs from 40% to 60% as recommended by procedure A. All other provisions of Test Method G133, procedure A have been followed. 2.5. Statistical analysis For the microhardness, wear volume, depth and wear rate data, the means and standard deviations were calculated using their classic formula for the sample mean and standard deviations. Similarly, the 68% and 95% confidence intervals for the means with lower and upper bounds were calculated and provided in the wear rate graphs. The t-student distribution for a degree of freedom of 3 (number of tests 4) was used for calculating p-values (%) for comparisons between coatings. The coefficient of variance (COV) for each given test condition/coating was calculated based on the calculated means and standard deviations. The two-sample independent t-test was used to determine whether there was a statistically significant difference between the means (calculated wear rates) for two unrelated coatings. 2.6. Microstructural characterization

2.4. Wear tests ASTM G-133-10 - Standard for Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear- Procedure A was used to test the sliding

The transmission electron microscopy (TEM) of the powders was done using a Tecani F20-200 kV (The Netherlands) in bright field (BF) mode. A JSM-8200 JEOL micro-probe (Tokyo, Japan) scanning electron

Fig. 2. WDS element maps for Ni + WC/Al2O3 (0.5 mol).

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by Parida et al. [19]. An image processing software program, ImageJ, was used for binarization and particle volume count of the element maps. The results for particles content measurements are shown in Table 2. An example of the element maps for the coatings (Ni + WC/Al2O3– 0.5 mol) is shown in Fig. 2. 3.3. Coating microhardness As described in Section 2.3, the average microhardness for each type of coating was measured using a minimum of 15 readings and the results are provided in Fig. 3. The upper and lower bars indicate the level of variation in microhardness measurement for each type of coating. Fig. 3. Average microhardness measurements for coatings.

microscopy (SEM) was used for the images of the cross-sections. The chemical composition and particle count were achieved using x-ray wavelength dispersive spectroscopy (WDS) - Quantax probe from Bruker Corporation (USA). A FEI/Philips XL30 (The Netherlands) scanning electron microscope in the field-emission mode (FE-SEM) was used for the images of the wear tracks.

3.4. Wear test results 3.4.1. Wear volume calculations It shall be noted that only the flat coated specimen wore during the sliding wear tests and no measureable wear was observed on the alumina balls used during the tests, and therefore the calculated wear volumes reflect the wear of flat coated specimens. Per formula provided in ASTM G-133, the wear volume of the flat specimens (Vf in mm3) is calculated from:

3. Results Vf ¼ AL

ð2Þ

3.1. Powder characterization The images from transmission electron microscopy (TEM) of the powders (Fig. 1) confirmed that the powders of both WC and Al2O3 are indeed nano-sized, however agglomeration of particles (due to electrostatic forces between them) caused formation of lager clusters in the TEM images. To reduce the agglomeration of the as-received particles (as a result of electrostatic force or unwanted compaction due to handling), an ultrasonic bath machine was used and the powders were subjected to sonication prior to adding them to the electrolyte solutions. Then the mixture was stirred by magnetic means for 12 h prior to the co-electrodeposition. 3.2. Characterization of coatings The x-ray WDS element mapping of the cross-section of the coatings with respect to tungsten, nickel and aluminium were obtained for particle content measurements. The particle content measurements of incorporated particles in the nickel matrix were done using element maps for aluminium, tungsten and nickel based on a method described

where A is the average cross-sectional area of the wear track (in mm2) and L is the length of the stroke (in mm). The cross-sectional area of the wear tracks was measured using the profilometer at three locations (one location at the center of the track and two locations each approximately 2 mm away from the center). The cross-section areas from wear profiles were calculated by numerical integrations using MATLAB programming. The length of the stroke was also measured from the worn specimens using optical microscope. The wear volumes and the statistical analyses for the test parameters (per guidelines from ASTM G-133) are provided in Fig. 4 and Table 3. 3.4.2. Wear rate measurements The wear depth progression as a function of time due to sliding wear was recorded using a LVDT sensor. The real time depth progression measurements for the coatings under each applied load are presented in Figs. 5 to 7. The increase in wear depth during the ball-on-plate wear tests is shown on the vertical axis (in μm) and time is shown on horizontal axis (in seconds). Based on the obtained wear rates for 4 measurements at each test condition (coating type and applied load),

Fig. 4. Calculated wear volumes for coatings.

3.8276 3.4598 2.4738 2.1642 1.2759 1.2806 0.7995 0.8798 15.7% 14.4% 16.1% 13.3% 0.4010 0.3424 0.2631 0.2018 0.1310 0.1596 0.0528 0.0844 2.6828 2.5298 1.6894 1.6064

2.5518 2.3702 1.6366 1.5220

1.5053 1.4032 1.2189 1.1701 0.7168 0.6389 0.4283 0.3494 11.2% 11.8% 15.1% 17.0% 0.1239 0.1201 0.1242 0.1290 0.0529 0.0722 0.0687 −0.0003 1.1639 1.0932 0.8923 0.7595

1.1111 1.0210 0.8236 0.7598

1.2596 0.8258 0.8731 0.7927 0.4680 0.4254 0.3294 0.3183 14.4% 10.1% 14.2% 13.4% 0.1244 0.0629 0.0854 0.0745 0.8638 0.6256 0.6012 0.5555 0.024 0.064 -0.011 0.025 0.888 0.690 0.590 0.581

405

the independent t-test was used to compare the wear rate for each coating to the rest of the coatings to verify whether the compared two coatings are from the same population and behave similarly under a given load (meaning that the difference in compositional characters due to particles volume difference has no impact on wear rates under a specific load). The independent two sample t-test results are provided in Table 4. The smaller p-value (in %), the less likely that the calculated means come from the same population, meaning that there is significant statistical evidence that the two coatings perform differently under the specific load (which can be caused by their difference in microstructure, microhardness or particle volume percent in their matrices). A greater p-value means there is a greater statistical chance that the difference in wear rate means between the two coatings is due to variability in the process and not necessarily as a result of differences in coating characteristics such as microstructure, microhardness or particle volume percent differences. For instance, when wear performance of Ni + WC (0.25 mol) is compared to Ni + WC (0.50 mol) under 15 N, the p-value is small, therefore it indicates that the difference in calculated wear rates is most likely caused by their characteristic differences. However, when the load is increased to 25 and 60 N, the calculated wear rates are not significantly different, therefore the difference in calculated means can be caused by numerous sources of variations in process. 4. Discussion

0.4058 0.3348 0.3148 0.1916 2.9576 2.7050 1.9515 1.7136 0.0112 −0.0327 −0.0456 0.0038 2.5630 2.3375 1.5910 1.5258 −0.5480 −0.4617 −0.3220 −0.2799 4 4 4 4 Applied load: 60 N Ni + WC (0.25 mol) Ni + WC (0.50 mol) Ni + WC/AI2O3 (0.25 mol) Ni + WC/AI2O3 (0.50 mol)

2.0037 1.9085 1.3147 1.2421

0.1025 0.0924 0.1357 0.1446 1.2136 1.1134 0.9593 0.9044 0.0240 0.0071 −0.0684 0.0242 1.1351 1.0282 0.7552 0.7839 −0.1794 −0.1717 −0.1360 −0.1685 4 4 4 4 Applied load: 25 N Ni + WC (0.25 mol) Ni + WC (0.50 mol) Ni + WC/AI2O3 (0.25 mol) Ni + WC/AI2O3 (0.50 mol)

0.9317 0.8493 0.6876 0.5913

4 4 4 4

0.692 0.540 0.485 0.447

−0.172 -0.086 -0.116 −0.108

0.886 0.628 0.652 0.576

0.023 0.003 0.051 0.020

0.989 0.645 0.677 0.618

0.125 0.019 0.076 0.063

4.1. Microstructure analysis

Applied load: 15 N Ni + WC (0.25 mol) Ni + WC (0.50 mol) Ni + WC/AI2O3 (0.25 mol) Ni + WC/AI2O3 (0.50 mol)

Wear vol. Replicate 2 (mm3) Deviation from average (mm3) Wear vol. Replicate 1 (mm3) Num of replicates Coatings

Table 3 Wear volume results for the coated specimens using procedure A of ASTM G133-05 (2010).

Deviation from average (mm3)

Wear vol. Replicate 3 (mm3)

Deviation from average (mm3)

Wear vol. Replicate 4 (mm3)

Deviation from average (mm3)

Average (mm3)

Std. Dev. (mm3)

Coeff. of var.

95% C L (Lower) (mm3)

95% C L. (Upper) (mm3)

M.A. Farrokhzad, T.I. Khan / Surface & Coatings Technology 304 (2016) 401–412

Fig. 1 represents the transmission electron microscopy images of particles from WC and Al2O3 powders used for electrodeposition and Fig. 2 represents the element maps from the cross-sections of one of the coatings (Ni + WC/Al2O3–0.5 mol). The elements maps show a successful uniform co-electrodeposition of ceramic particles in the nickel matrix. For the electrolyte solutions containing only WC particles, it was also found that the amount of incorporated particles in the matrix has increased from 9.5 wt% to 12.5 wt% when the concentration of WC particles in electrolyte solution increased from 0.25 mol to 0.5 mol. However, when Al2O3 particles were introduced into the electrolyte solutions, the incorporation of WC particles decreased to 5.5 wt% and 6.5 wt% for 0.25 and 0.5 mol mixed WC and Al2O3 solutions respectively, while a greater incorporation of Al2O3 particles was observed. One logical explanation for this observation is that the co-electrodeposition process may have a greater affinity for deposition of Al2O3 rather than WC particles when both WC and Al2O3 particles are present in the electrolytes, however further experimental investigation is required to provide a better understanding of this effect. As the concentration of Al2O3 particles increased from 0.25 mol to 0.5 mol, the deposition of the particles in the matrix was also increased. Based on these observations it can be stated that the increase in total particle content in the matrix is positively correlated to the total concentrations (of both type particles) in the electrolyte solution. A closer examination of the cross-sections of the coatings revealed that Ni + WC coatings had higher level of porosity near the surface than the coatings containing alumina (see Fig. 8). WC is thermodynamically unstable and it can oxidize when added to aqueous solutions. During the oxidation reactions in the aqueous solutions, WC reacts with oxygen forming mainly WO3 compounds. WO3 can be hydrated to give negative OH groups which provides a negative surface charge. Subsequently atomic hydrogen can form on the electrodeposition sites. Eventually the generated atomic hydrogen evolves into hydrogen molecules (H2) and from hydrogen bubbles leading to the formation of a porous microstructure [20]. Fig. 8a reveals a porous microstructure for the Ni + WC coating. It is believed that the addition of the non-reactive Al2O3 nanosized particles in the electrolyte solution can disturb the evolution of hydrogen at the co-electrodeposition sites, leading to a reduction of porosity within the Ni + WC/Al2O3 coating thickness (Fig. 8b). Higher porosity within the coating thickness is

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Fig. 5. Examples of wear depth measurements as function of time (left) showing the 68% and 95% confidence intervals for the means (right) - applied load of 15 N.

known to be associated with a lower coating microhardness, resulting in greater wear rates when the coating is subjected to abrasion. Additionally, with a higher porosity comes a softer coating which allows some of the energy from the moving ball to spread sidewise which increases the wear track width instead of the depth, thus affecting the overall wear volume measurements.

4.2. Coating hardness The presence of WC and Al2O3 in the metallic matrix increases the microhardness of the cermet coating. Table 2 shows that a greater volume of nano-sized particles was deposited from solutions with both particles added into the electrolyte than the WC-only electrolyte solution. The measured average microhardness for coatings (Fig. 3) revealed that in general Ni + WC/Al2O3 coatings have a significantly greater microhardness values than Ni + WC coatings. The increasing in hardness is attributed to the combined effects of; (a) dispersion strengthening caused by the presence of hard particles in the matrix (the approximate hardness for WC is 2000 HV and for Al2O3 is 2300 HV) as well as; (b) reduction of grain size in the matrix (also known as grain refinement) caused by constant interruption of nickel grain growth by particle attacks at the depositing sites. Therefore based on Hall-Petch relationship for grain size effect, an increase in the hardness can be expected for the coatings [21]. The rule of mixture for composite materials can also be used to explain the effect of particle deposition on the coating hardness. Based on the rule of mixture, the hardness for composite coatings

(H Comp.) can be formulated to volume percent and the hardness of ceramics and metallic constituents [22–25]: H Comp: ¼ H M V M þ

n X

ð3Þ

HC V C

C¼1

HM and HC are respectively the hardness values for metallic matrix and the deposited ceramic particles. Similarly, VC is the volume fraction of the deposited ceramic particles in the matrix and VM is the volume fraction of the matrix (VC + VM = 1). Thus the rule of mixture for Ni + WC and Ni + WC/Al2O3 can be written as: H NiþWC ¼ H Ni V Ni þ H WC V WC

ðwhere V Ni þ V Wc ¼ 1Þ

ð4Þ

HNiþWC=Al2 O3 ¼ HNi V Ni     þ H WC V WC þ H Al2 O3 V Al2 O3 where V Ni þ V Wc þ V Al2 O3 ¼ 1

ð5Þ As can be seen from Table 2, the volume percent of deposited particles in the nickel matrix increases by increasing the concentration of the particles in the electrolyte solution. The measured volume percent of the incorporated particles in the matrix varies from 9.5 wt% to 17.0 wt%. A similar pattern for the microhardness of the coatings (Fig. 3) can be seen which shows that the measured coating microhardness is proportionally increased by increasing the volume percent of particles in the matrix. Since both tungsten carbide and alumina have relatively

Fig. 6. Examples of wear depth measurements as function of time (left) showing the 68% and 95% confidence intervals for the means (right) - applied load of 25 N.

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Fig. 7. Examples of wear depth measurements as function of time (left) showing the 68% and 95% confidence intervals for the means (right) - applied load of 60 N (the singular spikes from Ni + WC-0.5 mol data are filtered and the normalized value was included for the confidence interval calculations).

similar microhardness values, the increase in microhardness of the coatings could be simplified to increase in volume percent of deposited particles in the nickel matrix. In this case, a positive correlation can be drawn between the volume percent of the total deposited particles to the overall microhardness of the coating, where an increase in particle volume directly translates into an increase in microhardness of the coating. The microhardness measurements (Fig. 3) also reveal that the correlation between particle content in the matrix and their corresponding hardness is not linear and Ni + WC perform below a hypothetical expected hardness value when the of mixture is considered. The underperformance in microhardness of Ni + WC can be attributed to the presence of a greater volume of porosity which can reduce the hardness of coating as described in the previous section.

and 25 μm. Fig. 9a represents a typical initial stage wear performance. The steady state stage for the wear tests was constant and liner. The wear depths vs. time graphs (Figs. 5 thru 7) represent the steady state wear depth progression for this stage (shown as starting at second 400 and lasted until second 1800 on the horizontal axis). In some occasions, the coatings were not thick enough to last the 1800 second abrasive wear and therefore, the substrate was worn (Fig. 9b). The failed tests were repeated to ensure that the substrate was not exposed during the tests. For the steady state wear stage, the wear rates for all coatings (the average rates for all 4 tests per coating and per load) have been calculated and presented in Table 5.

4.3. Sliding wear performance of coatings

The microhardness of cermet coating is dependent on the volume percent of incorporated particles in their metallic matrix [4]. The wear volumes are calculated based on the guidelines provided in ASTM G133 and the result and the related statistical analysis are provided in Fig. 4 and Table 3. For the applied load of 15 N, the difference in the wear volume between the coatings is not significant (between 0.5555 and 0.8638 mm3). However when applied load was increased to 25 N, the different coating performances become more significant. A gradual decline in calculated wear volume among the coatings can be observed with the increase of incorporated particle volume in the matrix. At the applied loads of 25 and 60 N, Ni + WC/Al2O3 coatings performed significantly better than Ni + WC coatings which is in agreement with microhardness observations. Based on Archard equation an increase in applied load from 15 to 25 N should cause a 60% increase in wear volume, however this effect cannot be observed on calculated wear volumes (Fig. 4). The apparent deviation from Archard equation can be attributed to the profilometer precision limits and the small amount of volume calculated for the wear track under applied loads of 15 and 25 N. When the applied load was increased from 25 to 60 N (a 40% increase), the wear volume increased significantly and it became proportional to the applied load difference as given by Archard equation. The coefficient of variances (COV) for the wear tests has been between 10.1 and 17.0% which is expected for sliding wear tests in dry conditions [15]. The relatively narrow range for the reported COVs indicates the degree of consistency in wear test procedure, operation and measurement. Based on the information provided in Table 4, it is evident that an increase in the applied load will increase p-values between the compared coatings. This increase is expected as the higher load tends to generate more severe surface damage, hence they can cause an increase in the degree of variability among measured means. By considering the particle volume percent values (Table 2), it appears that a greater p-value difference was calculated when the compared two coatings have a

Archard equation provides a mathematical formulation for wear rate of materials [26]. According to Archard equation, the wear rate (Q) of a coating is directly proportional to the load (W), sliding distance (x) and inversely proportional to its hardness (H): Q ¼ k0

Wx H

ð6Þ

Note that k0 is called wear coefficient and is a material related constant. For the studied materials, the sliding distance was constant and therefore, only the effect of applied load and microhardness were considered. For the coating materials three stages of wear can be identified, the initial stage, steady state stage and substrate wear. In general, in the initial stage the wear rate is rapid and an unstable phase. The surface roughness and anomalies, contaminations, presence of voids (Fig. 8) and poor surface crystallization are believed to cause the rapid wear performance of the coatings. The analyses of the wear rate charts for the studied coatings indicated that the initial stage lasted approximately between 100 and 400 s after the tests were initiated. The wear depths progression related to the initial stage is measured to be between 10 Table 4 Independent two-sample t-test results (p-value in %) for wear rate means. Coating

15 N

25 N

60 N

Ni + WC (0.25 mol) vs. Ni + WC (0.5 mol) Ni + WC (0.25 mol) vs. Ni + WC/Al2O3 (0.25 mol) Ni + WC (0.25 mol) vs. Ni + WC/Al2O3 (0.50 mol) Ni + WC (0.50 mol) vs. Ni + WC/Al2O3 (0.25 mol) Ni + WC (0.50 mol) vs. Ni + WC/Al2O3 (0.50 mol) Ni + WC/Al2O3 (0.25 mol) vs. Ni + WC/Al2O3 (0.50 mol)

0.6% 0.2% 0.1% 15.5% 1.6% 18.2%

38.7% 1.7% 0.3% 25.1% 6.5% 9.2%

56.9% 8.6% 6.1% 18.5% 11.5% 55.8%

4.4. Wear volume analysis

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Fig. 8. SEM of cross-sections of Ni + WC (0.5 mol) and Ni + WC/Al2O3 (0.5 mol) coatings.

smaller difference in their total incorporated particles volume percent. For instance, when paired the Ni + WC coatings (0.25 mol vs. 0.5 mol) or paired Ni + WC/Al2O3 coatings (again 0.25 mol vs. 0.5 mol) are compared, greater p-values were calculated. For these two comparisons, the difference in incorporated particle volume is minimized. Similarly, the smallest p-value is associated with the compared two coatings when their difference in volume percent of particles is greater such as when Ni + WC (0.25 mol) coating is compared to Ni + WC/Al2O3 (0.5 mol). Also when the coatings from both groups are compared against each other (Ni + WC vs. Ni + WC/Al2O3), the pvalues remain relatively small as opposed to when the two coatings are from the same group. Based on the above observation it is statistically acceptable to state that the addition of Al2O3 particles in the nickel matrix of Ni + WC has significantly improved the wear performance of the Ni + WC coatings. Also it can be stated that a negative linear correlation can be constructed between the volume percent of incorporated particles in the matrix and the wear rate (meaning that an increase in particle content will cause a decrease in wear rate). 4.4.1. Effect of hardness on wear rate Thiemig and Bund have argued that microhardness (and thus wear performance) of the composite coatings is positively affected by higher amounts of the deposited particles in the metallic matrix [27]. Based on the Archard equation, the wear rate of a material is inversely proportional to its hardness. The measured average microhardness for the coating (Fig. 3) shows that the coatings containing only WC particles have lower microhardness than coatings containing both Al2O3 and WC. Similarly when the wear volume (Fig. 4) and wear rates (Table 5) are compared for a coating under specific applied load, the wear rates for coatings with only WC particles are greater than coatings that have both particles

in the matrix. Ni + WC (0.25 mol) has the lowest particle content (and microhardness) among the studied coatings and it also has the highest wear rate among them. Similarly, Ni + WC/Al2O3 (0.5 mol) coating has the greatest volume of deposited particles in the matrix (and highest microhardness) and it has the lowest wear rate. Thus, it can be safely argued that wear rate of coating is inversely proportional to the particle content (see Table 2) in the matrix. Chen et al. have shown that wear rate reduction for the cermet coatings can be attributed to the presence of ultra-hard ceramic particles and their strengthening effect in the metallic matrix. When the coating is subjected to wear, the ultra-hard particles resist the applied force better than the metallic matrix and therefore, the wear rate will reduce [11]. It has been shown that the gradual protruding of nano-sized ceramic particles from the metallic matrix can protect the metallic portion against the abrasive object and therefore it can delay the material removal from the matrix, hence lower wear rates can be achieved [28,29]. 4.4.2. Effect of applied load on wear rate As can be seen in the graph shown in Fig. 4 (wear volume) Table 5 (wear rate), a positive correlation between applied loads and wear volume and rates can be drawn which follows Archard equation. However, for each type of coating, the increase in wear rate is not fully proportional to the increase in the applied load. This non-linearity in wear rate to load response suggests that the wear coefficient (k0) can also be affected by the applied load. This nonlinearity of wear rates, and subsequently the wear coefficient (k0) to the applied load also suggests that wear mechanisms that take place during the sliding tests may have been affected by the magnitude of the applied load. Jeong et al. have discussed that the abrasive wear mechanism for ceramic-metallic composite coatings occurs in the following stages; first, the applied load on the abrasive

Fig. 9. The observed stage transitions during the wear tests; (a) initial stage to steady state stage and (b) steady state stage to substrate wear.

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409

Table 5 Wear rates (depth increase vs. time in μm/s) as a function of applied loads.

15 N

25 N

60 N

Ni + WC (0.25 mol)

0.0300

0.0379

0.1000

Ni + WC (0.50 mol)

0.0179

0.0329

0.0857

Ni + WC/Al2O3 (0.25 mol)

0.0143

0.0264

0.0571

Ni + WC/Al2O3 (0.50 mol)

0.0107

0.0207

0.0507

Coating

object (i.e. ball) deforms the surface. Then, the deformed surface will crack, fractured and delaminated and finally the debris will be removed from the surface by the motion of the ball [28]. Additionally, the reciprocating motion of the ball on the surface is cyclic loading in nature which can result in fatigue wear by forming micro-cracks. The micro-cracks formed on the surface as a result of fatigue mechanism will continue to grow. When joined together, they create larger cracks which eventually delaminate larger portions of the surface [30]. For the light applied loads, the delamination of surface can be caused by fatigue cracking, as well as galling or pitting. However, when the load and number of cycles increase, strain hardening can also take place on the surface of the coating [28,31]. The strain hardening mechanism will increase the localized microhardness of the matrix and influence the overall wear performance of the coating which can result in lower wear rates [32–34]. Fig. 10 represents the FE-SEM images of wear tracks for Ni + WC (top) and Ni + WC/Al2O3 (bottom) coatings both produced with 0.5 mol electrolyte solutions and tested under the applied load of 15 N.

The images show formation of micro-cracks, segmented spallation and separation on the surface for both types of coatings. The formation of cracks and spalling can be attributed to surface fatigue as a result of the cyclic load. As the number of cyclic load increases, the crack propagation occurs underneath the surface and eventually causes the separation and fragmentation of the coating [6,16]. The amount of microcracks is more noticeable on the Ni + WC than Ni + WC/Al2O3 coatings. The histogram charts (illustrated for each wear track on Fig. 10) reveals a higher spread of measured wear asperities for Ni + WC coating (with most of the data clustered around 35 μm and spread in a 26.25 μm range) than Ni + WC/Al2O3 coating (with most of depths clustered around 15 μm and spread in a 8.75 μm range). A higher spread of wear asperities (on x-axis) reflects a relatively rougher surface when compared to a smoother surface (the smoother a surface is, the less deviation from the surface finish level). Therefore, it can be concluded that surface of Ni + WC/Al2O3 coatings is significantly less affected by the applied load than Ni + WC coatings. The lower wear depth for

Fig. 10. The FE-SEM images of wear tracks and wear depth histograms for Ni + WC (top) and Ni + WC/Al2O3 (bottom) under applied load of 15 N.

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Fig. 11. The FE-SEM images of wear tracks and wear depth histograms for Ni + WC (top) and Ni + WC/Al2O3 (bottom) under applied load of 25 N.

Ni + WC/Al2O3 as compared to Ni + WC can also be attributed to their greater relative microhardness (see Fig. 3) and as explained by Archard equation.

Fig. 11 represents the FE-SEM images of wear tracks for Ni + WC (top) and Ni + WC/Al2O3 (bottom) produced with 0.5 mol electrolyte solutions and tested under the applied load of 25 N.

Fig. 12. The FE-SEM images of wear tracks and wear depth histograms for Ni + WC (top) and Ni + WC/Al2O3 (bottom) under applied load of 60 N.

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Fig. 13. Microhardness of the coating underneath of the wear tracks for the coatings tested - applied load of 60 N.

411

compressive loads, materials are being removed and the new surface material is being cracked and spalled. Whereas for the Ni + WC/Al2O3 coatings the degree of material removal is less. Since the Ni + WC coatings have lower incorporated ceramic particles in the nickel matrix than the Ni + WC/Al2O3 coatings, the coating can behave more plastically. Some wear mechanism regard the wear of softer materials as plastic shearing of the surface followed by detachment of the wear particles [35]. Gül et al., also have described a second mechanism which involves smearing off the wear debris on the surface by combined effects of deformation strengthening and fatigue cracking mechanisms which can simultaneously take place on the surface of the coating under heavier loads [6]. The images shown in Fig. 12 for Ni + WC suggest that both mechanisms can be applicable to describe the wear mechanisms of the coatings under 60 N applied load. 4.5. Analysis of wear tracks

As can be seen from these images, the initial cracks have developed to full score marks along the sliding direction for both coatings. The histograms for the images reveal that the spread of wear depth is wide but they are clustered around 48 μm for the Ni + WC coatings (with a spread in the 22.25 μm range) and 25 μm for the Ni + WC/Al2O3 coatings (with a spread in the 8 μm range). Overall it can be observed that coatings containing both alumina and tungsten carbide particles show a greater wear resistance against the sliding ball as opposed to coatings containing only tungsten carbide particles. Fig. 12 represents the FE-SEM images of wear tracks for Ni + WC (top) and Ni + WC/Al2O3 (bottom) both produced with 0.5 mol electrolyte solutions and tested under the applied load of 60 N. From the wear depth histograms it is clear that the wear depth for Ni + WC/Al2O3 coatings is significantly (50%) lower than Ni + WC coatings (65 μm vs. 130 μm). Also, the wear track morphology of the two coatings is significantly different. Cracks, deep score marks and material removal are visible on the surface of the wear track for the Ni + WC coating but the surface of the wear track for the Ni + WC/Al2O3 coating (bottom) appears to be less affected. The smoother surface for Ni + WC/Al2O3 under 60 N when compared to the surface of the wear track for the same coating under 15 N and 25 N loads (Figs. 10 and 11) can be attributed to the greater compressive force of 60 N which can cause strengthening hardening of the coating surface. The heavier loads tend to squeeze and flatten out the surface asperities that are generated during the reciprocal motion of the ball, resulting in smoother surfaces. However, the same effect of flattened and smooth wear tracks under 60 N load cannot be seen on the SEM images of the Ni + WC coatings. The difference in wear track morphology between these two coatings under the 60 N load suggests that the wear mechanisms taken place on their surfaces could be different. For the Ni + WC coatings under heavy

Ni + WC coatings have lower microhardness when compared to Ni + WC/Al2O3 coatings. However, as the SEM images of the wear tracks revealed, the wear mechanisms that were taken place on the surfaces of these two types of coatings under heavy loads were different. To study the effect of applied load on the microhardness of the wear track, the cross-section of the coatings under the wear tracks subjected to the applied load of 60 N were examined and the microhardness measurement results are shown in Fig. 13. For this purpose, 10 micro-indentations were planted approximately 10 μm below the surface of the wear tracks (the samples were sliced at the center of the wear tracks and then polished prior to microhardness measurements). By comparing the results between microhardness before and after wear tests (Fig. 3 vs. Fig. 13), it is evident that the microhardness values for Ni + WC coatings before and after wear tests have remained unaffected, however the microhardness of the Ni + WC/Al2O3 coatings have been noticeably increased. A close-up examination of the surfaces of the wear tracks of the two coatings for the applied load of 60 N revealed that the surface morphologies of the wear tracks are also significantly different. Based on the close-up images shown in Fig. 14a, the surface of the Ni + WC coatings is ragged and the material was removed constantly and micro-crack formation was an ongoing process. The ragged surface of coating in the image (Fig. 14a) suggests that fatigue can be the dominating fracture mechanisms for the Ni + WC coatings under heavy loads. However, the surface of Ni + WC/Al2O3 (Fig. 14b) shows a relatively smoother surface which suggests that materials on the surface was removed due to eventual delamination of the surface by abrasive wear rather than fatigue cracking [35]. Because the wear rates are lower for the Ni + WC/Al2O3 coatings than the Ni + WC coatings, it can be stated that the kinetic energy from the ball has caused a greater hardness (due to strengthening hardening effect) of the wear track. On the contrary for

Fig. 14. Closed-up images of the wear track morphology, Ni + WC (left) and Ni + WC/Al2O3 (right) under applied load of 60 N.

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the Ni + WC coatings, the kinetic energy has caused more material removal than strengthening hardening. 5. Conclusions This paper investigates the ball-on-flat sliding wear performance (ASTM G-133) of two novel cermet coatings consisting of WC and a mixture of WC and Al2O3 co-electrodeposited into a nickel matrix. The following was concluded based on the test results and the discussion provided above: - An increase in particle concentration in the electrolyte solution resulted in an increase in the amount (volume percent) of deposited ceramic particles in the nickel matrix. - The microhardness of the produced coatings was positively correlated to the amount and hardness of the deposited ceramic particles (following the rule of mixtures for composites). - An addition of Al2O3 particles in the WC electrolyte solution increased the total entrapment of ceramic particles, resulting in cermet coatings with less porosity and higher hardness. - The independent two-sample t-test for wear rate means showed that the volume of incorporated particles in the matrix was a reliable indicator for coatings wear resistance. - Archard equation provided a good theoretical explanation for the effects of hardness and applied loads on wear behaviour of cermet coatings. - The wear mechanism for lighter loads was dominated by fatigue wear. For the heavier loads, the eventual delamination of the surface by abrasive wear was the dominant wear mechanism. The difference in wear mechanisms between the light and heavy loads was attributed to the potential strengthening hardening effect under heavier loads. - As applied load increased, the microhardness underneath the wear track also increased which supports the strengthening hardening hypothesis under heavier loads.

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