Effect of casting technique on surface roughness and consequent mass loss after polishing of NiCr and CoCr base metal alloys: a comparative study with titanium

Statement of problem

Surface roughness of cast metal frameworks may lead to difficulties in finishing or polishing procedures and weaken the framework.

Purpose

The aim of this study was to assess the surface roughness of 2 base metal alloys, submitted to different casting techniques, to determine the influence of surface roughness on loss of mass after polishing compared to commercially pure titanium castings.

Material and methods

Forty disk-shaped wax patterns (8 × 2 mm) were randomly assigned to 5 groups (n=8): (1) CoCr alloy (Wironit), acetylene-oxygen flame casting (WFC), (2) NiCr alloy (Verabond II), acetylene-oxygen flame casting (VFC), (3) CoCr alloy, vacuum casting (WVC), (4) NiCr alloy, vacuum casting (VVC), and (5) commercially pure Ti alloy (Ti) melted by electric arc in argon gas atmosphere. WFC and VFC served as controls. Both NiCr and CoCr alloys were invested in phosphate-bonded investment (Termocast). Ti was invested in ammonium dihydrogen phosphate (Rematitam Plus). The wax patterns and casting techniques were standardized. All specimens were cleaned with airborne-particle abrasion (aluminum oxide). Surface roughness (Ra) was assessed using a surface-test analyzer at 3 different sites. To evaluate the loss of mass, the specimens were weighed on a precision balance, then polished with 150-600 grain sandpaper until clinically acceptable (minimum roughness of 0.09 μm). The specimens were then reweighed. A 1-way analysis of variance (ANOVA) and Tukey HSD test (α=.05) were performed.

Results

Statistical analysis showed that VVC and WVC had significantly (P=.0050; P=.0057) smoother surfaces (2.43 ± 0.53 and 2.23 ± 0.49, respectively) than VFC and WFC (2.99 ± 0.44 and 2.83 ± 0.61, respectively), but were not significantly different from Ti (2.49 ± 0.62). The loss of mass (in %) was not significantly different for any group (3.18 ± 0.72, 3.14 ± 0.93, 3.36 ± 1.05%, and 4.14 ± 1.28% for VVC, WVC, VFC, and WFC, respectively). The mass loss of Ti was 4.32 ± 1.16.

Conclusion

Within the limitations of this study, the base metal alloys submitted to vacuum casting showed decreased surface roughness, similar to that of titanium, compared to base metal alloys submitted to acetylene-oxygen flame casting. There were no significant differences in loss of mass after polishing for all tested specimens.