Study On Pressureless Sintering And Annealing Of Silicon Nitride Ceramics With Y₂O₃-MgO Sintering Additives

As high-power and ultra-high-power integrated circuits are widely used in rail transportation and communications and aerospace,the reliability of packaging substrates is becoming more important.Silicon nitride ceramic are preferred for packaging substrate used in harsh environments due to its excellent mechanical properties and thermal conductivity.But the relatively high cost of preparation limits its application.Therefore,how to reduce its preparation cost while maintaining its excellent overall performance is a problem that needs to be solved for large-scale commercial applications.Based on the demand of reducing the preparation cost of silicon nitride ceramics,relatively inexpensive Si powder andα-Si₃N₄powder were selected as the raw material powder,Mg O and Y₂O₃ were used as sintering additives and ZrO₂ was used as nitriding catalyst.Silicon nitride ceramics were prepared by nitriding treatment and pressureless sintering process.The effects of Si powder content and sintering holding time on the microstructure,phase composition,mechanical properties and thermal conductivity of the silicon nitride ceramics were investigated.Based on it,the effect of the variation of ZrO₂ content on the microstructure,phase composition,mechanical properties and thermal conductivity of silicon nitride ceramics was investigated.Samples selected for the characteristics of ZrO₂ variable were annealed to study the effects of different annealing temperatures on the structure and properties of silicon nitride ceramics.The main study contents and conclusions were as follows.The silicon nitride ceramics were prepared by using different proportions of Si powder and Si₃N₄ powder as raw materials and holding them for different times in pressureless sintering process.The composite sintering additives for silicon nitride ceramics were Mg O and Y₂O₃,and ZrO₂ was the nitriding catalyst.As a comparison,the other silicon nitride ceramics were also prepared by usingα-Si₃N₄ powder as raw material and Mg O and Y₂O₃ as composite sintering additives and the same process.The effects of Si powder content and holding time on the structure and properties of silicon nitride ceramics were studied.The results showed that the grains grew as the increase of holding time,the Si powder content and holding time affected the samples’phase composition.The addition of Si powder is beneficial to the improvement of the densities and mechanical properties of the samples.When the Si powder content was 15%（calculated by the mass of Si powder nitrided into silicon nitride as a percentage of the total mass of silicon nitride）and under the 6-hour holding condition,the three-point flexural strength of the sample was 826 MPa,the fracture toughness was 8.6 MPa·m^1/2,and the thermal conductivity was 61 W/（m·K）.Compared to the sample without Si powder addition,its flexural strength was improved by more than 15%and fracture toughness was improved by more than 30%.The effect of small variations in ZrO₂ content on the structure and properties of the samples was investigated on a base formulation with the Si powder content of 15%.The results showed that the addition of ZrO₂ was beneficial to the grain growth of silicon nitride ceramics to a certain extent.The variation of ZrO₂ content had a significant effect on the intergranular phase of the sintered silicon nitride ceramics,and a positive correlation was observed between its addition and the thermal conductivity of silicon nitride ceramic.The mechanical properties showed an increasing and then decreasing trend with the increase of ZrO₂ addition.In terms of phase composition,mechanical properties and thermal conductivity,the ZrO₂ content has a range of process windows.When the ZrO₂ additions were 1.5～1.8 wt%（15Si-10Z and 15Si-12Z）,the samples had similar comprehensive properties.The Vickers hardness was 14.8 GPa and 14.9 GPa,the fracture toughness was 8.6MPa·m^1/2 and 8.5 MPa·m^1/2,the flexural strength was 827 MPa and 826 MPa,and the thermal conductivity was 59.56 W/（m·K）and 60.99 W/（m·K）,respectively.Based on the above study,silicon nitride ceramic samples with 1.8 wt%（15Si-12Z）of ZrO₂ and 2.1 wt%（15Si-14Z）of ZrO₂ were selected for a 2-hour annealing treatment in the medium temperature range（1275°C～1500°C）.The effect of annealing temperature on the phase composition and properties of silicon nitride ceramics was studied.The results showed that annealing promoted the conversion of the intergranular glassy phase into the crystalline phase and induced the conversion of the crystalline phase.With the increase of annealing temperature,the mechanical properties of samples showed a decreasing trend,and the increase of ZrO₂ content suppressed the negative effect of annealing on mechanical properties to some extent.The annealing treatment was beneficial to the enhancement of thermal conductivity and the decrease of dielectric loss of the sample.The 15Si-14Z sample annealed at 1350°C had good overall performance with Vickers hardness,fracture toughness,flexural strength,thermal conductivity,breakdown resistance and dielectric loss of 14.4 GPa,7.9 MPa·m^1/2,762 MPa,65 W/（m·K）,23.1 KV/mm and 0.0011,respectively.In this study,silicon nitride ceramics with good mechanical,electrical and thermal conductivity were successfully prepared at low cost by using Si powder and silicon nitride powder as raw materials and pressureless sintering process.It is a useful reference for the study of low-cost preparation of silicon nitride ceramic substrate.Through further research,it is expected to realize the pressureless sintering preparation of higher-performance silicon nitride ceramic substrate and broaden their application fields.