Oxidation Microstructures and Interfaces in the Oxidized Zirconium Knee

This contribution describes the microstructural development of a predominantly zirconia scale resulting from thermal oxidation of a Zr–2.65 wt% Nb alloy that is used to provide a superior resilient bearing surface on the femoral condylar component in a novel total knee prosthesis. The oxide scale exhibits superior tribological properties articulating against a polyethylene tibial component, resulting in substantially reduced wear debris and attendant inflammatory response. Critical attributes of this scale are biological compatibility, hardness, maintenance of surface smoothness during articulation, and adhesion to the substrate. Thermogravimetry, transmission electron microscopy, and in situ hot‐stage X‐ray diffraction analyses of the oxide scale and underlying alloy substrate were employed to reveal the microstructural origins for the efficacy and integrity of this oxidation surface treatment. The wrought alloy in the condylar component exhibits a two‐phase microstructure comprising elongated hexagonal α‐Zr grains (<1 wt% Nb) discontinuously surrounded by cubic β‐Zr (∼18 wt% Nb) sheath grains. During oxidation for times up to 8 h in air at temperatures above and below the eutectoid at 620°C, interface control of oxidation kinetics occurs at an alloy/scale interface that advances into the alloy by inward oxygen diffusion. The oxide scale predominantly comprises columnar grains of monoclinic ZrO 2 , ∼40 nm wide × 200 nm long, with [001] m and [111] m fiber textures and arranged in a brickwork pattern that is highly resistant to lateral fracture and surface grain pull‐out that would compromise scale integrity and function. The scale forms under substantial compression, up to nearly −2 GPa in the surface plane throughout the oxidation, and maintains its compressive state when cooling down to room temperature. At the scale/alloy interface, unoxidized Nb stringers in oxidizing β‐Zr second‐phase grains extend from the alloy into the oxide scale and appear to anchor the scale to the alloy, accounting for the excellent scale adhesion observed. Further oxidation of the β‐Zr second phase farther into the scale is associated with isolated outcrops of equiaxed oxide enhanced in segregated Nb content. Nearer the scale/air interface, lathlike separation into Zr‐rich and Nb‐rich oxides and lamellar intergrowths of a mixed‐oxide phase (probably 6ZrO 2 ·Nb 2 O 5 ) occur but do not appear deleterious to the integrity or functionality of the scale, especially as the outermost portion of the scale is removed in establishing the final bearing surface of the commercial prosthesis.