Depends upon specifics, Silk pillowcases are generally permissible for women. However, for men, it is not permissible to use natural silk for sleeping or covering, just as it is forbidden for men to wear natural silk. The use of silk by women is not prohibited, as concluded by various Islamic scholars. Allah knows best.

Sources:

• IslamQA – Is it prohibited in Islam to sleep on silk?
• IslamWeb – Women may use silk pillowcases