Several standards govern the in-vitro characterization of porous coatings. ASTM standards are listed below for each type of testing, and there are FDA guidance documents that also provide helpful information, Guidance for Industry on the Testing of Metallic Plasma Sprayed Coatings on Orthopedic Implants to Support Reconsideration of Postmarket Surveillance Requirements and Guidance Document for Testing Orthopedic Implants with Modified Metallic Surfaces Apposing Bone Or Bone Cement. The test methods below are often adapted to evaluate other surface modifications and enhancements designed for bony ingrowth or other cementless fixation technologies.
Abrasion testing is performed per ASTM F1978 (Standard Test Method for Measuring Abrasion Resistance of Metallic Thermal Spray Coatings by Using the Taber Abraser) to quantify abrasion resistance of metallic coatings produced by thermal spray processes on flat metallic surfaces. This testing may also be adapted for evaluating wear for other orthopedic surface modifications (additive manufacturing of porous structures, metallic coatings on PEEK substrates, etc.). Samples are typically tested in ambient air with the Taber Dual Abraser with a specified abrading wheel weight and for a predetermined number of cycles. Wear modes are observed using high resolution microscopy, and cumulative mass loss is quantified after 2, 5, 10, and 100 cycle intervals with sub-milligram resolution. A customized sample drying process may be used to improve consistency of test results.
ASTM F1044 (Standard Test Method for Shear Testing of Calcium Phosphate Coatings and Metallic Coatings) and F1160 (Standard Test Method for Shear and Bending Fatigue Testing of Calcium Phosphate and Metallic Medical and Composite Calcium Phosphate/Metallic Coatings) outline shear testing of continuous calcium phosphate coatings and metallic coatings adhering to dense metal (Titanium, Cobalt Chrome) and plastic (PEEK, PAEK, UHMWPE) substrates. Shear fatigue testing determines the adhesive or cohesive strength, or both, of the coating. Shear fatigue strength is typically assessed in ambient air at predetermined test frequency, load ratio and coating shear area. Specimens are bonded to metal or plastic substrate dummies, placed into shear fixtures and tested to 10M cycles or failure. Fatigue testing may consist of either developing a S-N stress-life curve or completing several run-out tests against predetermined acceptance criteria. Pre-fatigue static testing per ASTM F1044 can be used to determine starting stress levels for fatigue testing.
ASTM F1147 (Standard Test Method for Tension Testing of Calcium Phosphate and Metallic Coatings) describes tensile testing of calcium phosphate and metallic porous coatings adhering to dense metal substrates. Static tensile strength is determined at pre-specified environmental conditions (temperature, humidity), loading rate (in displacement control), and coating area. Coated specimens are bonded to metal or plastic substrates, mounted on a test frame and loaded to failure.
Rotating beam fatigue testing per ASTM F1160 determines the effects of surface modifications, such as plasma spray, on the bending fatigue strength of metallic and plastic materials. It can also be used to assess calcium phosphate coatings, additive manufactured materials, and other surface enhancement technologies. The bending fatigue test quantifies the coating’s effects on the substrate it is applied to. Test programs are typically aimed at developing S-N curves with multiple specimens tested at each stress level. Testing is performed at up to 50 Hz for up to 10 million cycles.
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Shear Fatigue Fixture