For materials which are hard to sinter a process called liquid phase sintering is commonly used. Materials for which liquid phase sintering is common are Si3N4, WC, SiC, and more. Liquid phase sintering is the process of adding an additive to the powder which will melt before the matrix phase. The process of liquid phase sintering has three stages:

1. Rearrangement - As the liquid melts capillary action will pull the liquid into pores and also cause grains to rearrange into a more favorable packing arrangement.
2. Solution-Precipitation - In areas where capillary pressures are high(particles are close togeather) atoms will preferentially go into solution and then precipitate in areas of lower chemical potential where particles are non close or in contact. This is called "contact flattening" This densifies the system in a way similar to grain boundary diffusion in solid state sintering. Oswald ripening will also occur where smaller particles will go into solution preferentially and precipitate on larger particles leading to densification.
3. Final Densification - densification of solid skeletal network, liquid movement from efficiently packed regions into pores.

For liquid phase sintering to be practical the major phase should be at least slightly soluble in the liquid phase and the additive should melt before any major sintering of the solid particulate network occurs, otherwise rearrangement of grains will not occur. From : www.wikipedia.org