Influence of spray distance on mechanical and tribological properties of HVOF sprayed WC-Co-Cr coatings
Published Online: Apr 21, 2022
Page range: 545 - 554
Received: Feb 02, 2022
Accepted: Mar 01, 2022
DOI: https://doi.org/10.2478/msp-2021-0047
Keywords
© 2021 Monika Górnik et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
In the field of surface engineering, thermal spraying is a well-known technique employed in order to deposit various types of protective coatings that are used in a wide range of industrial branches [1,2,3]. Depending on the application, different coating manufacturing processes could be selected: flame [4, 5], arc [6, 7], plasma [8, 9] or cold spray [10, 11]. Among them, high-velocity oxy-fuel spraying (HVOF) is the primary method used to deposit hard, wear-resistant coating that exhibits both corrosion and erosion resistance [12, 13]. In this technique, gas combustion is used as an energy source that melts and accelerates powder particles.
Construction of the gun enables in-flight particles to achieve supersonic velocity (up to 2,000 m/s). On the other hand, the flame temperature is much lower than for plasma jet (c.a. 3,500 K instead of c.a. 15,000 K) [14]. These conditions make it possible to obtain coatings with very low porosity levels and low oxidation factors, as well as high adhesion to the substrate [15, 16]. Moreover, this method allows depositing both ceramic and metallic coatings, as well as cermet ones [17,18,19]. In addition, even polymers could be deposited by the HVOF technique, but it requires a very short residence time in the flame. Such coatings are used in dry sliding wear [20]. Nevertheless, among all the material types, the most frequently used is cermet. An additional advantage of thermal spraying is the possibility of replacement electrolytic hard chromium plating (EHC). This technology is considered environmentally hazardous, especially in industrial-scale applications [21, 22].
In general, manufactured coatings derived from the hard metals and their composites are used in extreme work conditions, e.g. impeller shafts, conveyor screws, pump housings etc. [14]. One of the most commonly used feedstock materials in the HVOF process is WC-Co, or wider, WC-based cermet. Such materials demonstrate very good wear resistance [23]. The most fundamental types of feedstock are WC-12 wt% Co, WC-17 wt% Co and WC-12 wt% Ni. These coatings have been intensively investigated for many years [24,25,26]. Development of different industry branches forced researchers to search for materials exhibiting improved properties. Among others, the addition of chromium to the cobalt matrix was found to improve the wear resistance of deposited coatings [27]. Nevertheless, it requires adjusting the appropriate process parameters, as well as the adequate chromium content. Among many parameters that strongly influence the quality and properties of manufactured coatings, spray distance is one of the key parameters. Moreover, during industrial conditions, this factor can be changed most easily, in contrast to e.g. gas flow rate [28, 29].
In this work, WC-Co-Cr (with 10 wt% Co and 4 wt% Cr) coatings manufactured by HVOF with different spray distances were characterised in terms of their microstructure, phase composition and mechanical and tribological properties, including adhesion, microhardness and sliding wear resistance. So far, the influence of spray distance in WC-Co-Cr coatings has been investigated only in a small number of articles. Such research has been carried out e.g. by Hong et al. [30], Lee and Kim [31] and Gui et al. [32].
The feedstock material used in the current investigation was a commercial powder of tungsten carbide with the addition of chromium in cobalt matrix (Amperit 554.071 by Hoganas). The nominal chemical composition was (in wt%) WC – 86, Co – 10 and Cr – 4. The morphology of the initial powder is presented in Figure 1. The powder particles were sintered and it was ensured that the particle size did not exceed 60 μm, according to the manufacturer's instructions. Own measurements of the particle size distribution (PSA 1090, Anton Paar, Graz, Austria) confirmed this, and the mean volume to surface diameter (d
Fig. 1
Morphology of initial feedstock power (SEM-SE) with random measurements of particle size. SEM, scanning electron microscope
Coupons of mild steel S235JR, with thickness 3 mm and diameter 25 mm, were used as the substrate material. Before spraying, the substrate surface was degreased and sand-blasted under a pressure of 0.6 MPa using alumina powder (mesh 40). The roughness of the substrate surface was determined using a MarSurf PS10 profilometer (Mahr, Esslingen, Germany) as an average of five measurements. Just before the deposition process, the substrates were cleaned using an ultrasonic bath.
Cermet coatings were sprayed using a CJS K5.2 N (Thermico) gun in the HVOF method. The scheme of the deposition process is presented in Figure 2. The powder feed rate was 30 g/min and it was delivered by a carrier gas (nitrogen) with a flow rate of 12 l/min. Flow rates for fuel media were as follows: kerosene – 16 l/h, oxygen – 600 l/min and hydrogen – 100 l/min. Linear gun velocity was 800 mm/s. The sample code and values of spray distance are collected in Table 1.
Fig. 2
Schematic representation of the HVOF method and its key process parameter, inspired by Pawłowski [3] and Jonda and Łatka [33]. HVOF, high-velocity oxy-fuel spraying
Values of spray distance and sample code
| 240 | 280 | 320 |
The metallographic samples were prepared by a standard procedure, which is as follows: samples were mechanically cut, embedded in epoxy resin and then ground and polished. The microstructure of manufactured coatings was analysed using a scanning electron microscope (SEM), Tescan Vega 3 (Tescan Orsay Holding).
The porosity of deposited coatings was analysed using ImageJ open source software (1.50i version). For each sample, 20 images were taken at a magnification of 1,000×. These images also have been obtained using SEM.
X-ray diffraction (XRD) investigations were performed using the D8 Advance diffractometer (Bruker, Karlsruhe, Germany). The set-up was equipped with a Cu-K
The pull-off adhesion test was carried out using an Elcometer 510 tester (Elcometer Instruments, Manchester, UK), according to the ISO 14916 standard. The samples were glued to the counter-surface with cold cure epoxy adhesive Epidian 53, with a theoretical strength of 70 MPa. Pins having a diameter of 10 mm and a load rate of 0.4 MPa/s were used during testing.
The dry sliding wear tests were carried out using the ‘pin-on-disc’ method in accordance with the ASTM G99 standard with a linear mode tribometer, version 6.1.19 (Anton Paar, Peseux, Switzerland). Before tests, coatings were ground and polished in order to obtain a surface roughness (
An example of a sprayed surface topography is presented in Figure 3. Some irregularly shaped WC grains can be observed. This is a result of melting of the matrix during flying in the flame. On the other hand, WC grains with a high melting point were not dissolved in the flame [37, 38].
Fig. 3
The topography of the A1 sample surface
The microscopic observation of coatings of all cross sections revealed a relatively smooth and dense structure (Figure 4 left column). Such compact and uniform structure results from inherent features of the HVOF method (high kinetic energy of the particles). Moreover, it could be observed that at the coating–substrate interface, the coating material had filled the unevenness of the substrate surface, and good interlocking with the substrate had occurred [39,40,41]. All coatings have been deposited in order to obtain c.a. 100 μm in thickness.
Fig. 4
SEM images of coatings’ cross sections:
Detailed examination at higher magnifications (Figure 4 right column) shows a low level of porosity with fine pores, whose size is lower than 3 μm. The calculated porosity of A1, A2 and A3 coatings is 2.5 ± 0.4, 3.3 ± 0.6 and 3.9 ± 0.7, respectively. Similar results have been obtained in the literature [42, 43]. It should be stressed that similar results of coatings porosity for the same material and with nearing spray distance were obtained by Murugan et al. [44].
Phase compositions of the deposited coatings are presented in Figure 5. In all coatings, hexagonal WC (PDF#00-061-0244) was the main phase. The second phase, hexagonal W2C carbide (PDF#00-035-0776), appeared. Thus, it is evident that during coating deposition, partial decarburisation of WC has taken place. It is a well-known mechanism for tungsten carbide coatings [45, 46].
Fig. 5
XRD patterns of all WC-Co-Cr coatings manufactured by the HVOF method. XRD, X-ray diffraction; HVOF, high-velocity oxy-fuel spraying
On the other hand, Cr addition to WC-Co slows down the decarburisation of WC. Moreover, Cr reinforces the cobalt matrix in the binding function of WC grains [47]. The last phase identified was a cubic solid solution of tungsten in cobalt with composition Co0.9W0.1 (PDF#03-065-9928). The index of carbide retention (ICR) has been proposed by Bartuli et al. [48] as a tool with which to determine the influence of process parameters on the degree of coatings’ decarburisation. This indicator compares the intensity of the (100) peak of WC at 2
Measurements of hardness instrumented indentation (HIT) revealed a slight influence of spray distance (Table 2). All coatings are characterised by hardness values between 14 GPa and 15 GPa and the values are not significantly different within the error range. These results are comparable with the results of other investigations [19, 51]. In the case of elastic modulus, it could be noted that the lowest EIT value is connected with the longest spray distance. For other samples, there is no such visible tendency and the obtained results are consistent with the literature [52, 53].
Values of elastic modulus and hardness from instrumental indentation test
| |
14.82 ± 1.25 | 14.67 ± 1.55 | 14.24 ± 1.35 |
| |
339 | 342 | 336 |
The tensile adhesion tests were carried out to estimate bond strength. The results are shown in Table 3. The influence of spray distance is visible from the results. However, the differences between values are only a little more than the limits of the standard deviation. Such results are comparable with the literature [54, 55]. For all samples fracture type was an adhesive-cohesive one.
Mean values of bond strength and type of fracture
| |
65 ± 3 | 61 ± 4 | 55 ± 3 |
| A/C | A/C | A/C |
A/C, adhesive-cohesive type of fracture
The results of dry sliding wear investigations in form of wear factor are given in Table 4. The obtained results confirmed that the deposited coatings exhibit very good wear resistance. Moreover, the slight influence of spray distance is also observed. Similar results can be found in the literature [41, 51, 56].
Sliding wear results: wear factor and friction coefficient (CoF)
| |
77 ± 11 | 73 ± 9 | 81 ± 14 |
| 0.41 ± 0.07 | 0.40 ± 0.09 | 0.40 ± 0.07 |
In order to understand the wear mechanism, the worn surfaces of investigated samples were examined by SEM. The wear traces of tested samples are presented in Figure 6. Identification of wear mechanism and occurred phenomena should take into account, that it is affected by two phases structure of cermet materials (hard grains and soft matrix). On the other hand, the wear resistance of HVOF sprayed coatings depends on many factors, e.g. microstructure, hardness, cohesion in the coating and porosity level [57]. The detailed analysis of wear traces of the tested coatings leads to the conclusion that two types of mechanism occurred: (i) subsurface plastic deformation and (ii) brittle cracking [51]. It should be stressed that these two mechanisms occur simultaneously. Due to the friction between two bodies, shear stresses occur, which makes possible the plastic flow of the soft metal matrix. On the other hand, during the brittle cracking, coating material detachment has occurred. Mainly, hard carbides particles are removed [58]. In all samples, the specific friction layer is built on their surface (Figure 6).
Fig. 6
SEM images showing the wear track of deposited coatings:
It contains mixed ingredients of the coating material (darker smooth areas) and less densely compacted wear debris, which are also oxidated (bright smooth areas) due to an increasing temperature during the test [59, 60].
Summarising all the conducted investigations and their results, some recommendations related to specific areas of application can be provided. In general, a shorter spray distance is recommended for applications that require improved corrosion resistance as it is connected with low porosity and a more homogenous structure. A slightly higher spray distance could be recommended for applications where good wear and erosion resistance are important. Too short a spray distance results in breaking the powder particles instead locking in surface unevenness. Moreover, for cermet materials, an excessive melting of carbides could take place. On the other hand, too long a spray distance leads to a reduction in velocity and insufficient particle deformation. In both cases, the porosity level significantly increases.
In this paper, HVOF spraying with cermet feed-stock powder having the chemical composition WC-10Co-4Cr was carried out. The coatings were studied in terms of the influence of spray distance on the microstructure and phase composition, as well as mechanical and tribological properties. The following findings are summarised:
All coatings were successfully deposited; moreover, in each sample, the interface between coating and substrate was clear. There was no evidence of delamination or other discontinuities. SEM micrographs revealed dense, compact and homogenous structure with a relatively low level of porosity, in the range 2.5–3.9 vol.%. XRD phase analysis revealed the presence of hexagonal WC, hexagonal W2C car-bide and a cubic solid solution of tungsten in cobalt with the composition Co0.9W0.1. Moreover, longer spray distances result in a slightly higher ICR (ICR = 0.95) instead of 0.93 for the shortest spray distance. Instrumental indentation hardness (HIT) shows a clear tendency with spray distance, which is as follows: HIT The main tensile adhesion of all coatings to the substrate was 65 MPa, 61 MPa and 55 MPa for A1, A2 and A3 samples, respectively. It is connected with the coating porosity and very good mechanical interlock between coating and substrate. The wear rate for all HVOF coatings did not exceed the level of 10−7 mm3/(N · m). The best wear resistance has been achieved for sample A2 (average spray distance) and it was probably caused by a beneficial mix of compact structure, high hardness and stiffness and high ICR (0.95). The CoF value (approximately 0.4 for all samples) indicates that no seizure occurs between the surfaces that are in contact. The wear mechanism includes plastic deformation and brittle cracking.