Mobile phone glass hardness testing measures the scratch resistance, indentation hardness, and abrasion durability of cover lens materials used in smartphones, tablets, and wearables. By verifying performance against recognized standards such as ASTM D3363, ASTM E384, and ISO 9385, manufacturers can validate ion-exchange strengthening processes, control incoming material quality, and substantiate durability claims. This technical guide covers sample requirements, test items, test methods, and practical applications for hardness testing of mobile device cover glass.
Technical Overview
Modern smartphone cover glass is typically a thin (0.3–0.7 mm) alkali-aluminosilicate sheet strengthened by a chemical ion-exchange process, in which smaller sodium ions at the glass surface are replaced by larger potassium ions in a molten salt bath. The result is a compressive surface layer—commonly 30–60 µm deep, with surface compressive stress in the range of roughly 500–900 MPa for high-grade materials—enclosing a tensile core. Hardness testing quantifies how effectively this engineered surface resists permanent deformation and scratching from everyday hazards such as keys, coins, and airborne quartz dust, which has a Mohs hardness of 7.
No single number fully describes the "hardness" of phone glass. The industry therefore relies on several complementary measurement scales: the Mohs scale (qualitative scratch ranking), pencil hardness by the Wolff–Wilborn method (semi-quantitative and widely used in commercial specifications), Vickers and Knoop microindentation (quantitative load-versus-indent-size measurements), instrumented indentation at the nanoscale (essential for thin coatings), and functional abrasion tests such as steel wool, falling sand, and rotary wheel abrasion that simulate real-world wear. A complete characterization program combines at least one intrinsic hardness method with one or more functional scratch and abrasion methods.
- Material class: chemically strengthened aluminosilicate glass, with sapphire and glass-ceramic used for premium and camera-lens applications
- Typical intrinsic hardness: approximately 6–7 GPa (roughly 550–700 HV0.1) for strengthened cover glass; sapphire is substantially harder
- Primary standards bodies referenced: ISO, ASTM, GB/T, JIS, and EN
- Testing is normally performed in ISO/IEC 17025 accredited third-party laboratories or in manufacturer process-control laboratories
Testing Samples
Finished Cover Lenses (2D, 2.5D, and 3D Formed Parts)
Complete cover lenses, including flat 2D parts, edge-polished 2.5D parts, and deep-drawn 3D curved parts, are tested to evaluate the surface as delivered to the end user. Curved regions are acceptable for pencil and scratch testing provided a flat zone of at least 20–25 mm exists at the point of contact; microindentation requires a small locally flat area and stable fixturing. Anti-fingerprint (AF or oleophobic) coated production parts should be tested both as received and after coating removal if the intrinsic glass hardness must be isolated.
Raw Sheet Glass and Panels
Unprocessed strengthened sheet glass, cut into coupons of typically 50 mm × 50 mm or 100 mm × 100 mm, is used for process development and incoming quality control. Testing coupons eliminates variability from cutting, grinding, and polishing operations and allows indents to be placed well away from edges, where residual stresses and chips distort results.
Coated and Treated Specimens
Specimens carrying anti-fingerprint fluorosilane coatings (typically 10–20 nm thick), anti-glare textured surfaces, or print layers are tested to determine how the treatment affects scratch and abrasion response. Because such coatings are far thinner than the indentation depth of standard microhardness methods, coated samples are evaluated mainly by pencil hardness, nanoindentation, scratch testing, and abrasion testing rather than by conventional Vickers or Knoop measurements.
Alternative Cover Materials
Sapphire crystal covers, glass-ceramic sheets, and hybrid laminate structures submitted for comparative benchmarking follow the same hardness test matrix. Sapphire, with a Mohs hardness of 9, requires higher indentation loads or nanoindentation protocols, while glass-ceramics require careful selection of load to avoid radial cracking around the indent.
Sample Preparation and Conditioning
Samples must be free of pre-existing scratches, chips, and contamination in the test area, and should be cleaned with isopropyl alcohol and lint-free wipes before testing. Conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 24 hours, consistent with ISO 291 or equivalent ASTM conditioning practice, is recommended before testing. A minimum of three to ten specimens per lot is customary so that statistical scatter, which is significant in brittle materials, can be assessed.
Testing Items
Hardness test programs for mobile phone glass are assembled from the following measurement items, selected according to the specification being verified and the failure mode of interest.
- Mohs scratch hardness on the 1–10 mineral scale
- Pencil hardness on the 9B–9H scale (Wolff–Wilborn method)
- Vickers microindentation hardness, typically reported as HV0.05 to HV0.1
- Knoop microindentation hardness, typically reported as HK0.1 or HK0.5
- Instrumented indentation hardness and elastic modulus per ISO 14577
- Single-pass scratch resistance, including critical loads for first cracking and coating delamination
- Steel wool abrasion resistance at defined load and cycle count
- Falling sand or gravel abrasion resistance
- Rotary wheel (Taber-type) abrasion evaluated by haze and light transmission change
- Anti-fingerprint coating durability, evaluated by water contact angle retention after abrasion
Testing Methods
Pencil Hardness Test (Wolff–Wilborn Method)
Pencil hardness is the most widely cited figure in commercial specifications for phone glass. Drafting pencils graded from 9B (softest) to 9H (hardest) are prepared by abrading the lead flat, mounted in a holder at 45 degrees to the surface, and pushed across the glass under a 750 g load for a stroke of approximately 6.5 mm. The reported rating is the hardest pencil grade that does not produce a visible scratch or mar at a defined number of repetitions. The governing procedures are ASTM D3363, ISO 15184, GB/T 6739, and JIS K5600-5-4, all originally developed for organic coatings and adapted by the display-cover industry. Strengthened cover glass typically rates between 6H and 9H, but the rating is strongly dependent on applied load, pencil condition, and operator technique, so full test conditions must accompany any reported grade.
Mohs Scratch Hardness Test
Mohs testing uses reference mineral points—from talc (1) through gypsum (2), calcite (3), fluorite (4), apatite (5), feldspar (6), quartz (7), topaz (8), and corundum (9) to diamond (10)—drawn across the surface under light hand pressure. The Mohs hardness is the hardness of the hardest mineral that does not leave a permanent scratch. Typical strengthened cover glass falls between 6 and 6.5, while sapphire rates 9. The test is qualitative but highly relevant to field failures, because common abrasive grit is largely quartz, explaining why glass surfaces scratch despite high pencil-hardness ratings.
Vickers Microindentation Hardness
The Vickers test presses a square-based diamond pyramid with an included face angle of 136 degrees into the polished surface under a low load—commonly 50 gf to 1 kgf for cover glass, with HV0.05 and HV0.1 typical reporting conditions—held for 10 to 15 seconds. Both diagonals of the recovered indent are measured optically, and hardness is calculated as HV = 1.8544 · F / d², where F is the load in kgf and d is the mean diagonal in mm. The method follows ASTM E384, with ASTM C1327 and ISO 6507-1 providing ceramic and metallic counterparts. Five or more indents are averaged, indents must be spaced at least 2.5 diagonal lengths apart, and the indent depth should remain below roughly one tenth of the specimen thickness and within the compression depth so that the strengthened layer itself is measured. Strengthened aluminosilicate cover glass typically measures 550–700 HV0.1.
Knoop Hardness Test
The Knoop test uses an elongated rhombic diamond indenter with an approximate long-to-short diagonal ratio of 7:1, producing a shallow, narrow indent that is particularly well suited to brittle glass because it minimizes radial cracking. Hardness is calculated from the long diagonal as HK = 14.229 · F / L². The procedures most applicable to phone glass are ASTM C730, the standard test method for Knoop indentation hardness of glass, and ISO 9385 for glass and glass-ceramics. Typical values for cover glass lie around 450–600 HK, while sapphire exceeds 1900 HK.
Instrumented Indentation (Nanoindentation)
Instrumented indentation per ISO 14577-1 records load and penetration depth continuously at loads from micronewtons to millinewtons, with penetration depths below about 200 nm. This allows direct measurement of the indentation hardness and elastic modulus of thin anti-fingerprint coatings, textured surface skins, and the ion-exchanged layer itself. Cross-sectional indentation mapping can profile how hardness varies with depth through the compression layer, providing valuable feedback for ion-exchange process optimization.
Single-Pass Scratch Testing
Scratch testing draws a diamond stylus—commonly a Rockwell C cone with a 200 µm tip radius—across the surface under a constant or progressively increasing load from about 1 N to 30 N over a 5–10 mm track. Friction force, acoustic emission, and post-test microscopy are used to identify critical loads: the load at which the first cohesive crack appears and, for coated specimens, the load at which adhesive delamination of the coating begins. ASTM C1624, developed for advanced ceramic coatings, and constant-load scratch approaches in the style of ISO 1518 are adapted for glass cover evaluation. Critical-load values provide a quantitative, repeatable ranking of scratch resistance that complements pass/fail pencil ratings.
Steel Wool Abrasion Testing
Steel wool abrasion is the industry-standard functional wear test. A pad of grade 0000 (4/0) steel wool, typically 25 mm × 25 mm, is pressed against the surface under a normal load of 750 g to 2 kg and reciprocated for a defined number of cycles, often 500 to 5,000. Acceptance criteria usually require no scratch visible to the unaided eye under specified illumination, a haze increase below an agreed limit, and, for AF-coated parts, a retained water contact angle above approximately 100 degrees after testing. Because no single international standard governs the procedure for cover glass, the load, cycle count, and evaluation criteria must always be stated in the test report.
Falling Sand and Rotary Abrasion Testing
Falling sand abrasion per ASTM D968 flows standardized silica or silicon carbide grit from a height of about 914 mm onto an inclined specimen, with abrasion resistance expressed as the volume of abrasive required to wear through a defined depth. Rotary wheel abrasion in the manner of ASTM D1044 uses a CS-10F or similar abrasive wheel under a 500–1000 g load and reports the increase in haze and loss of light transmission after defined cycle counts. Both methods simulate gritty, particle-driven field wear and are particularly informative for anti-glare and textured surfaces.
Applications
Hardness and abrasion test data support decisions across the entire product lifecycle of mobile device cover glass.
- Incoming quality control: verifying that each lot of strengthened sheet glass meets agreed pencil hardness, microhardness, and steel wool abrasion acceptance limits before fabrication
- Ion-exchange process validation: correlating bath time and temperature, compressive stress, and depth of layer with measured hardness and scratch critical loads during process development and DOE studies
- Material benchmarking: comparing aluminosilicate, soda-lime, lithium-aluminosilicate glass-ceramic, and sapphire options on a consistent quantitative basis
- Claim substantiation: providing laboratory evidence behind retail hardness and durability claims, with documented test conditions, for regulatory and marketplace compliance
- Failure analysis: distinguishing quartz-grit scratches, coating abrasion, and impact damage from manufacturing defects when analyzing returned or warranty units
- Coating development: measuring wear life and contact angle retention of anti-fingerprint and anti-glare treatments under standardized abrasion
- Adjacent markets: extending the same test matrix to smartwatch covers, tablet glass, automotive center-stack displays, and AR or VR optical windows
FAQ
What pencil hardness rating should good phone glass achieve?
Well-strengthened aluminosilicate cover glass typically rates 7H to 9H under a 750 g pencil load applied per ASTM D3363-style conditions. Ratings below 6H generally indicate either an insufficiently strengthened surface or test conditions that are more severe than the specification assumes. Because results depend heavily on load and pencil preparation, buyers should always require the full test conditions, not just the grade.
Is a "9H" rating the same as Mohs hardness 9?
No. The two scales are not comparable. A 9H pencil-hardness result describes resistance to the hardest drafting pencil under a specific load, while Mohs 9 corresponds to corundum and sapphire. Most cover glass has a Mohs hardness of only about 6 to 6.5, which is why ordinary quartz sand (Mohs 7) can still scratch a screen advertised as 9H.
How many samples are required for a hardness test program?
Microindentation methods require a minimum of three to five indents per sample on at least three samples, while pencil, scratch, and abrasion methods generally require three to ten complete specimens per condition to characterize scatter. Additional samples are recommended whenever coated and uncoated states, or multiple load levels, are compared.
Is hardness testing destructive?
Yes. All indentation, scratch, and abrasion methods leave permanent marks or wear tracks on the specimen. Testing should therefore be performed on sacrificial coupons or on production parts that will not be shipped.
Which indentation load is appropriate for thin cover glass?
Loads of 50 gf to 100 gf (HV0.05 to HV0.1) are preferred for phone glass. Higher loads produce deeper indents that can punch through the ion-exchanged compression layer and approach the specimen thickness, underestimating the strengthened surface hardness and increasing the risk of radial cracking that invalidates the diagonal measurement.
How long does a typical hardness test program take?
Standard pencil, Mohs, Vickers, and Knoop testing is usually completed within three to five working days. Programs that include scratch mapping, steel wool and Taber abrasion sequences, contact angle measurement, and report review typically require five to seven working days at accredited third-party laboratories.
Which standards combination is recommended for a comprehensive evaluation?
A common specification package combines ASTM D3363 or GB/T 6739 for pencil hardness, ASTM E384 or ISO 6507 for Vickers hardness, ASTM C730 or ISO 9385 for Knoop hardness on glass, ISO 14577 for coating-level nanoindentation, and a defined internal steel wool procedure with stated load, cycles, and acceptance criteria. This combination covers intrinsic hardness, coating response, and functional wear in a single coherent report.