Quantitative Assessment of Micro-Vickers Measurement Precision in Relation to Surface Roughness

Surface quality is crucial for precise micro-Vickers hardness measurements. Current standards provide limited quantitative guidance on acceptable surface roughness levels. This study examines the impact of varying surface roughness levels on measured hardness values and measurement accuracy for two materials: aluminium alloy 6082-T651 and stainless steel Uddeholm Ramax HH. For each material, six samples were prepared with distinct surface roughness levels (Sa from ~ 0.02 to 0.72 µm) and characterised by 3D surface texture measurements. Microhardness tests were conducted across a range of applied loads from 0.005 to 1 kgf. Higher surface roughness (Sa > ~0.2 µm) led to increased measurement variability, and larger differences in indentation diagonal lengths—effects most pronounced at test loads ≤ 0.1 kgf—whereas ISO 6507-1:2023 precision (coefficient of variation < 5%) was consistently achieved when roughness was reduced to Sa ≤ 0.05 µm for the aluminium alloy and Sa ≤ 0.07 µm for the stainless steel. A quantitative threshold based on the ratio of roughness to indentation depth (Sa/h) is proposed, offering practical limits for surface preparation. These findings highlight the importance of surface quality and provide new insights for enhancing the reliability of microhardness measurements in materials characterisation.