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 ZrO2, ∼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 6ZrO2·Nb2O5) 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.

Oxidized Zr-2.5Nb is being developed as an articular bearing surface for the femoral component in total joint arthroplasty. It has so far demonstrated superior wear performance against ultrahigh molecular weight polyethylene (UHMWPE) with respect to traditional articulating materials such as Co-Cr-Mo alloys. In this investigation, we used thermogravimetric analysis, transmission electron microscopy, and in situ x-ray diffraction techniques to study the microstructure and stress state of the oxide scale grown on Zr-2.5Nb.The oxidation temperature not only determines the kinetics of oxidation but the morphology of the various oxidation products. We have identified the oxidation products of both phases of the two phase alloy and correlated them with the original alloy microstructure. These include not only monoclinic zirconia but also small amounts of tetragonal zirconia and a mixed oxide phase combining both zirconium and niobium. The alloy microstructure both influences the final oxidation products and is reflected in the microstructure of the oxide. The oxide scale itself has a predominantly columnar microstructure which extends from the oxide/metal interface to the outer surface of the oxide. In situ x-ray diffraction measurements revealed that the oxide scale is stressed in compression following cooling and exhibits strong crystallographic texture. The oxide/metal interface is continuous, without pores or voids which might be detrimental to oxide adhesion. In addition, we have identified a phase which develops at the interface between the beta-zirconium grains and the oxide. We have also identified amorphous regions within the oxide scale which serve as sinks for silicon and other impurity elements found in the alloy.