Influence of carbide morphology

Influence of carbide morphology on abrasion wear resistance and carbide dissolution was studied by mixing dense-coated WC, spherical fused WC/W2C/W and angular fused WC/W2C with Metco 12C (30 HRC ~ 304 HV). These carbides had nearly equal size distributions, which excluded the influence of carbide size on dissolution and wear behaviour.

All three different coatings were laser clad with identical parameters. Abrasion wear results and microhardness values are summarized in Table 20. Representative micrographs taken from the transverse cross-sections (perpendicular to cladding direction) together with high magnification SEM micrographs taken from the wear scars are illustrated in Figures 108-112.

Wear scar for Metco 12C + Amperit 522.2 (70/30 vol.%) was essentially similar to one displayed in Figure 101. For that reason, it is not shown here.

Table 20. Influence of carbide morphology on abrasion wear resistance and microhardness.

Coating Volume 1) volume ratio according to image analysis

2) volume loss calculated on the basis of average matrix/carbide volume ratio in coating measured with image analysis

3) matrix microhardness 4) carbide microhardness

According to image analysis conducted on transverse cross-sections (perpendicular to cladding direction), average primary carbide volume fractions varied from 24 (SFTC) to 38%

(Woka 9604). Fortunately, volume fractions of dense-coated WC and angular fused WC/W2C were nearly the same (35 vs. 38 vol.%) enabling to make some comparative study. On the basis of mass and volume losses, it is evident that coatings reinforced with angular fused WC/W2C exhibited more than twice better abrasion wear resistance than coating reinforced with dense-coated WC. This substantial difference in resistance can be explained by comparing the microhardness values and wear scars. Bulk microhardness of the coating reinforced with angular fused carbides was clearly higher (820 vs. 678 HV1). This partly originates from the slightly higher volume fraction of primary carbides and higher matrix hardness but mainly from the difference in behaviour of the primary carbides under the indentation load. Namely, dense-coated WC particulates cracked severely under the both indentation loads (10 and 3 N) as already explained in section 3.4.1.1. Some of those carbides were cracked along grain boundaries even without any external load (except sample preparation) as illustrated in higher magnification micrograph in Figure 108. With a load of 3 N average microhardness of the dense-coated WC was just 1225 HV0.3 compared to 1950 HV0.3 for angular fused carbides. Wear scar studies revealed that dense-coated WC particulates cracked also more severely under used abrasive conditions than angular fused ones. If the matrix microhardness values are compared, matrix reinforced with angular fused carbides exhibited higher values (568 vs. 650 HV0.3). This suggests that angular fused WC/W2C carbides dissolved more abundantly to matrix than dense-coated WCs. XRD curves measured from both of the coatings (Figure 113a and c) confirmed this as the integrated areas of unnamed peaks, which belong presumably to precipitated mixed carbides are larger for

coating reinforced with angular fused carbides.. It is known that W2C is chemically [222] and thermodynamically less stable than WC (at least temperatures below ~1400C˚) (Figure 9). For this reason it may have had higher degree of dissolution in Ni-based matrix. Also angularity and associated sharp corners may have enhanced the dissolution further. This dissolution behaviour is in good agreement with results reported in Refs. [223, 224, 391]. Wear behaviour, however, contradicts with rubber wheel abrasion wear results reported by Huang et al. [224]. In their study, coating reinforced with less dissolved WC particulates exhibited better abrasion wear resistance than coating reinforced with WC/W2C and angular WC particulates survived the test without fragmentation. Their crushed WC particulates were produced by carburisation of elemental W without subsequent dense-coat processing.

Manufacturing process of WC particulates used here before dense-coating process is unknown but commercially the carburisation of elemental W is the most important and thus the most probable method in this case. Luft et al. [391] used visually and compositionally identical powder in their studies and they called it agglomerated and dense-coated.

Figure 108. Optical micrographs taken from the transverse cross-section (perpendicular to cladding direction) of Metco 12C + Amperit 522.2 (70/30 vol. %) coating. Some of the WC particles were cracked along grain boundaries even without any additional mechanical load (except sample preparation).

Figure 109. Optical micrographs taken from the transverse cross-section (perpendicular to cladding direction) of Metco 12C + WC/W2C/W SFTC (70/30 vol.%) coating.

Figure 110. SEM micrographs taken from the wear scar of the Metco 12C + WC/W2C/W SFTC (70/30 vol.%) coating. Rubber wheel rotation direction was from left to right. Some of the carbides are split in two in plane parallel to surface. Majority of the carbides are, however, intact. Signs of plastic flow are seen on the surface of matrix.

Figure 111. Optical micrographs taken from the transverse cross-section (perpendicular to cladding direction) of Metco 12C + WC/W2C Woka 9604 (70/30 vol.%) coating. Carbide microstructure consists of WC needles (dark) in W2C matrix (light).

Figure 112. SEM micrographs taken from the wear scar of Metco 12C + WC/W2C Woka 9604 (70/30 vol.%) coating. Rubber wheel rotation direction was from left to right. Carbides are much more severely damaged than SFTC ones but not as badly as dense-coated WCs.

Comparison between fused carbides (angular vs. spherical) is more difficult because of different volume fractions of primary carbides in coatings (24 vs. 38 vol.%). This difference may have originated from the incomplete mixing of powders or mechanical segregation in powder hopper since microhardness values do not indicate any excessive dissolution of spherical carbides, which would have decreased the primary carbide content. Assuming that matrix and carbides exhibited uniform wear, volume losses suggest that spherical fused carbides are better because they showed just slightly higher volume losses (18.6 vs. 16.5 mm³) despite notably lower volume fraction of carbides. SEM micrographs taken from the wear scars, however, reveal that matrix has worn down much more profoundly in coating reinforced with spherical carbides. This increases the difference in real wear volumes since wear volume of coating reinforced with spherical fused carbides increases. The reason for this higher degree of matrix wear in coating reinforced with spherical carbides can be found from dissolution. Angular fused carbides may have dissolved more severely due to sharp corners as revealed by large difference in matrix hardness values (570 vs. 650 HV0.3) and the integrated areas of unnamed peaks in Figure 113. On the other hand, it may have also resulted exclusively from the higher carbide content (24 vs. 38 vol.%). According to WC/W2C volume fraction calculations based on integrated areas of peaks WC

( )

^{1011 vs. W2C}

( )

^{1011 and}

WC

( )

^{1120 vs. W2C}

( )

¹¹²⁰ , amount of WC with respect to W2C increased in both coatings.

WC/W2C for spherical fused WC/W2C/W powder was 40/60, whereas in coating it was 45/55. In angular fused WC/W2C powder relation was 43/57 and in coating 60/40. Both of

these examples indicate the faster dissolution of W2C over WC and higher susceptibility of angular fused ones to dissolution. Concerning carbide hardness, it was observed that spherical fused carbides exhibited significantly higher hardness than angular fused ones (2420 vs. 1950 HV0.3). This resulted almost equal bulk hardness values (810 vs. 820 HV1) despite notably lower carbide content. Wear scar images also speak for spherical carbides since they remained predominantly intact, whereas angular ones cracked. Spherical shape together with high hardness and existence of W could explain this difference in cracking. For instance, spherical ones may have softened the impact of abrasives by receiving them in less steep/high angles even if the attack angle is not so crucial in three-body abrasive wear where abrasives are free to rotate. And cubic (bcc) W could have increased their fracture toughness.

2θ 2θ

2θ

Figure 113. XRD patterns measured from a) Metco 12C + Amperit 522.2 (70/30 vol.%), b) Metco 12C + SFTC (70/30 vol.%) and c) Metco 12C + Woka 9604 (70/30 vol.%) coatings.

In document Engineering coatings by laser cladding - the study of wear and corrosion properties (sivua 157-161)