ASTM C730 Knoop Hardness Testing for Glass

ASTM C730 Knoop Indentation Hardness Testing Overview

ASTM C730 measures the Knoop indentation hardness of glass using a precision Knoop diamond indenter. The indenter is an elongated diamond pyramid with a length-to-width ratio of 7.11:1. It is pressed into a polished glass surface under a defined load between 100 gf and 1,000 gf, held for 10 to 15 seconds, and the long diagonal of the residual indent is measured optically. The Knoop hardness number is calculated as HK = 14.229 × P / d², where P is the applied load in kgf and d is the long diagonal in mm. Results are reported with the applied load, for example, “560 HK₁₀₀”.

Knoop is the preferred microhardness method for glass because the shallow, elongated indent generates far less radial cracking than a Vickers (square pyramid) indent of the same diagonal length. This makes Knoop suitable for thin glass plates, chemically strengthened glass with shallow compressive layers, optical coatings on glass substrates, and any application requiring intact indents for accurate diagonal measurements. ASTM C730 is the US standard for glass Knoop hardness. For ceramic whitewares, the companion standard is ASTM C849, and for advanced technical ceramics, it is ASTM C1326. Higher HK values indicate harder, more wear and scratch-resistant glass.