Local differences in wear rates

In order to reveal the possible local differences in wear rates, 13 different types of already tested laser coatings consisting of 4 consecutive layers were submitted to diamond wheel grinding, where they were levelled until the wear scar or no as-laser-clad surface remained.

Due to low peak-to-valley distances in vertical direction originated from the overlapping and low volume losses occurred during testing, layer thicknesses need to be removed were so low that the flat ground surface intersected the outermost clad layer. After the abrasion wear tests, surface line profiles were measured in direction perpendicular to cladding and wear direction using laser profiler. The length of the profile was 10 mm covering several reheated zones associated with overlapping. Some of the coatings were also cut transversely perpendicular to wear direction and cross-section samples were prepared. Abrasion wear test results of these diamond wheel ground surfaces are expressed as volume losses in Figure 134. Most of the

Figure 133. SEM micrographs taken from the wear scar of SHS 1389 coating. Rubber wheel rotation direction was from left to right. Incompletely melted or mixed single SHS 1378 particles are seen.

coatings exhibited lower volume losses when ground flat. WR6 and SHS 1378 laser coatings were tested twice. Abrasion wear resistance of the latter one approached to those of high volume fraction (~75 vol.%) WC-based cemented carbides. It is also notable that WR6 reinforced with externally added VCs outperformed substantially WR6 alone.

48.4 37.2 27.5 24.4 20.5 20.3 15 13.8 12.1 10.5 8.1 6.2 5.4

Figure 134. Rubber wheel abrasion wear results for diamond wheel ground laser coatings.

Results for WR6 (COV 6.9%) and SHS 1378 (COV 23.6%) are the average of two measurements. All the other coatings were tested only once. Volume losses for (Ti, Mo)C SHS laser coatings are slightly overestimated because Mo in carbides was not taken into account.

3.4.6.1 (Ti, Mo)C coatings

Surface profiles were measured from the tested SHS 1377 ((Ti, Mo)C – Ni (50/50 wt.%)) and SHS 1389 ((Ti, Mo)C – St 6 (20/80 wt.%)) coatings. Both of these coatings were laser clad with inter-track advance of 1.4 mm. Surface profile measured from SHS 1377 revealed very smooth peaks and large peak-to-valley distances in vertical direction, which were approximately 250 μm. Regular distance between valleys was approximately 1.2 mm and between peaks approximately 1.3 mm. Surface profiles measured from SHS 1389 revealed distinctively sharper peaks than in SHS 1377. Peak-to-valley distances in vertical direction were approximately 45 μm. Regular distance between valleys was approximately 1.4 mm.

These results suggest that overlapped areas (width of the valleys were narrower than widths of the peaks) exhibited higher wear rates than central parts of the beads.

3.4.6.2 Fe-based coatings

As explained earlier in section 3.4.4, WR6 laser coating consisted of harder reheated and softer non-reheated zones. Widths of the reheated zones were 0.7–0.8 mm, whereas widths of the non-reheated zones were 0.4–0.7 mm. According to surface profiles, valleys appeared at the regular interval of 1.4 mm, which was also the inter-track advance used in cladding.

Widths of the peaks (reheated zones) were 0.7–0.8 mm. Widths of the valleys (non-reheated zones) were 0.4–0.6 mm. This indicates that reheated zones exhibited better wear resistance than non-reheated zones as could be expected on the basis of microhardness values. Peak-to-valley distances in vertical direction were very low; 14 μm.

Similar to WR6, Nanosteel laser coating generated regular wear pattern. This coating was laser clad with inter-track advance of 1 mm. Optical micrographs revealed that peaks appeared at the interval of 1 mm and they were located at the overlapped regions. This suggests that reheated zones were hardened. Horizontal hardness measurements confirmed the increased hardness in reheated zones. These reheated zones were approximately 0.3 mm in width and 910 HV1 in hardness. Hardness of the non-reheated zone was 780 HV1. Average hardness of the coating was 840 HV1.

3.4.6.3 NiCrBSi + WC coatings

Surface profile studies were made for Metco 12C + Woka 9604 (70/30 vol.%), Metco 12C + WC SFTC (70/30 vol.%) and Metco 16C + recycled WC (70/30 vol.%) coatings. Inter-track advance in laser cladding was 1 mm for all the coatings. All the coatings except Metco 12C + WC SFTC (70/30 vol.%) generated regular wear pattern, which corresponded to the inter-track advance used. Representative example is shown in Figure 135, where regular wear pattern generated on the surface of Metco 12C + Woka 9604 (70/30 vol.%) coating. Valleys coincided well with overlapped regions, which suffered from lower primary carbide content similar to cases reported in Refs. [223, 225, 283, 392, 393]. Gassmann [225] suggested that Marangoni flow is not efficient enough to transport carbides to the remote edges of the bead, and thus homogenize the structure when high volume fraction of WCs is used. This together with excessive dissolution of carbide due to reheating could explain this. Such a low carbide content areas were not observed in Metco 12C + WC SFTC (70/30 vol.%) coating where the true carbide volume fraction was 24%.

3.4.6.4 CrC-based coatings

According to surface profile measured and cross-section prepared from Stellite 6 + CrC (65/35 vol.%) laser coating, distinct regular wear pattern generated. Valleys appeared at the distance of 1 mm, which corresponded to the inter-track advance used in cladding. It was already earlier mentioned in section 3.4.3 that coatings reinforced with CrC suffered from

excessive carbide dissolution in reheated zones. These valleys coincide well with the reheated zones.

Figure 135. Regular wear pattern on the surface of laser clad Metco 12C + WC Woka 9604 (70/30 vol.%). Distance between the valleys is 1 mm.