| [Objective]To investigate the effect of heat treatment under vacuum and nitrogen atmosphere on the mechanical properties and metal-ceramic bond strength of SLM cobalt-chromium alloys,and to compare its properties with raw specimens without heat treatment.[Material and methods]18 dumbbell standard tensile specimens,as well as 19 discs with 10 mm diameter and 1 mm height were prepared on each base plate(three base plates in total)by SLM technique.Tensile samples were fabricated with the axes deviating from the build direction by 0°(0°-specimens),45°(45°-specimens),or 90°(90°-specimens).According to the different heat treatment atmosphere,the above three base plates were divided into three groups,the raw specimens without heat treatment,heat treatment under nitrogen and vacuum atmosphere respectively,labeled as group As-built,group N2 and group Vac respectively.The specimens were heated to 1150? in a furnace under nitrogen and vacuum atmosphere and maintained this temperature for an hour.Separated all the specimens from the base plates using wire electrical discharge machining,then the tensile test and Vickers hardness test were carried out,to explore the differences in tensile strength,elongation at break,the reduction of area and Vickers hardness among the groups.Field emission scanning electron microscope were used to determine the fracture mechanism.Twelve discs were randomly selected from each of the three groups,and casting cobalt-chromium alloys were used as the control(group C).The specimens were polished under running water.After sandblasting,ultrasonic cleaning was carried out and dried at room temperature,followed by specific dental porcelains fusing on the surface of cobalt-chromium alloys.Shear bond strengths(SBS)test was conducted,and fracture surface analysis was also performed to determine the failure modes by stereomicroscope.The interface characteristics of the metal-porcelain were determined by field emission scanning electron microscope coupled with energy dispersive spectroscopy after metallographic preparation.SPSS 22.0 software was used to test the normality and homogeneity of variance of the data.Two-way ANOVA was used to analyze the results of tensile test,and one-way ANOVA was conducted to analyze the results of Vickers hardness test and shear strength test.The test level was ?=0.05.[Results]1.Effect of heat treatment atmosphere on mechanical properties of SLM cobalt-chromium alloyThe tensile strength,elongation at break,and the reduction of area of cobalt-chromium alloys prepared by SLM technique at different angles were significantly different among the three groups(P < 0.05).At the same angle,the tensile strength of group As-built was significantly higher than that of group N2 and group Vac,with the elongation at break and the reduction of area lower than those of group N2 and group Vac;For 0°-specimens,the tensile strength,elongation at break,and the reduction of area of group Vac were much higher than those of group N2;however,for 45° and 90° specimens,there was no significant difference in the tensile test results between group N2 and group Vac.The comparison of tensile strength among the three angles under the same conditions was as follows: in the group As-built,the tensile strength of 90°-specimens was significantly higher than that of 0°-specimens,while in group N2 and group Vac,the tensile strength of 90°-specimens was significantly lower than that of 0°-specimens.The comparison of elongation at break and reduction of area among angles was that: 0°>45°>90°.The difference of hardness value(MPa)after three heat treatment was statistically significant(P<0.05).The hardness value of group As-built was the largest(378.86±10.28)followed by group N2(346.57±11.02),while that of group Vac was the smallest(315.14±9.31).According to the analysis fracture mechanism after tensile test,group As-built-45° was classified as quasi-cleavage fracture,group As-built-90° was the brittle fracture,and the other groups are all the ductile fracture.2.Effect of heat treatment atmosphere on porcelain bond strength of SLM cobalt-chromium alloyThere was a statistical significance in SBS values(MPa)between group Vac(39.74±2.58)and group C(35.92±2.60)(P<0.05).The SBS values of the group As-built(37.68±1.15)and the group N2(37.79±3.37)was between the group Vac and the group C,but there was no statistical difference between themselves.In addition,both group As-built and N2 didn't show any significant differences compared to group Vac or group C.After SBS test,samples in the all test groups exhibited combined failure.The results of SEM showed that the metal-ceramic interface of the four groups were tightly integrated,with no obvious pores and cracks.EDS analysis showed that the element diffusion layer was actually visible in each group,among which the diffusion depth of Si elements in the group Vac wider than that in the group C.[Conclusion]1.Compared with the raw specimens without heat treatment,the tensile strength and hardness of SLM cobalt-chromium alloys decreased after heating to 1150?,for 1h with elongation at break and reduction of area increasing.2.The mechanical properties of SLM cobalt-chromium alloys heated treatment under vacuum atmosphere could be superior to those of specimens under nitrogen atmosphere.3.In this study,the heat treatment atmosphere showed no significant influence on the metal-ceramic bond strength of cobalt chromium alloys produced by SLM technique.4.The porcelain adhesion of SLM cobalt-chromium alloys could be comparable to that of casting cobalt-chromium alloys,while heat treatment under vacuum atmosphere could further improve the bonding strength between porcelain and cobalt chromium alloys. |
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