Abstract <p>This study investigated pore growth during stretching of polymerization-filled composites based on Al₂O₃ particles coated with ultra-high-molecular-weight PE, HDPE, or a dual-layer ultra-high-molecular-weight PE/HDPE coating. The particles were spherical in shape with an average diameter of 20 μm. When the composite was stretched, the particles exfoliated, creating pores. During stretching, the spherical pores were elongated, first becoming elliptical and then conical. The sharp angles of the rhomboid pores are due to the plastic flow of the polymer, not its degradation. Pore volume increases linearly with the material extension ratio. This increase in pore volume is due to the presence of rigid particles within the pores, which limit the lateral contraction of the composite. Bands representing transverse interactions between the particles and the polymer are visible in the equatorial region of the pores. Presumably, polymer elongation is greatest in these microregions and the polymer degradation process begins in them. It has been hypothesized that at a temperature of 190°C, the tensile stresses generated during composite pressing do not relax in the ultra-high-molecular-weight PE melt. This is explained by a molecular entanglement network.</p>