Hardness Testing
Abrasive hardness governs cutting efficiency, wear on blasting equipment, achievable surface profile, and breakdown rate. Harder abrasives produce deeper profiles and clean faster but wear equipment more rapidly. Two hardness scales are commonly used: Mohs (field screening) and Vickers/Knoop (laboratory precision).
Mohs Hardness — Field Assessment
The Mohs scale (1–10) is a relative scratch hardness ranking. It provides qualitative classification of abrasive materials and is widely used for field lot verification and incoming material screening. It does not provide an absolute hardness value and should not be used for engineering calculations.
| Abrasive Material | Mohs Hardness | Vickers (HV) | Relative Cleaning Rate |
|---|---|---|---|
| Steel shot (low carbon) | 5.5–6.5 | 390–530 | Moderate |
| Steel grit (high carbon, hard) | 7–8 | 700–900 | High |
| Cast iron grit | 7–8 | 700–950 | High |
| Garnet (almandine) | 7–7.5 | 1050–1300 | Very High |
| Copper slag | 6–7 | 600–800 | High |
| Staurolite | 7–7.5 | 900–1100 | High |
| Aluminum oxide (white) | 9 | 1800–2000 | Very High |
| Silicon carbide | 9–9.5 | 2400–2800 | Extremely High |
| Olivine sand | 6.5–7 | 700–900 | Moderate-High |
| Coal slag (black beauty) | 5.5–6.5 | 500–700 | Moderate |
Vickers Hardness — Laboratory Method
Individual abrasive particles are mounted in resin, polished cross-section, and indented with a diamond pyramid indenter at specified load (typically 0.1–1 kgf). The diagonal lengths of the resulting indentation are measured under microscope and converted to HV value. A minimum of 10 readings are taken per sample and averaged, with standard deviation reported. This method is used for lot qualification and specification verification.
Mounting medium: Conductive bakelite or epoxy resin
Polishing sequence: 240 → 400 → 600 → 1200 grit, then 1 µm alumina finish
Test load: HV0.1 (0.1 kgf) — fine particles; HV0.3 or HV1 — larger particles
Dwell time: 10–15 seconds
Number of readings: Minimum 10; report mean ± standard deviation
REJECT IF: Individual reading > ±15% of target specification mean
Bulk Density Testing
Bulk density is the mass of abrasive per unit volume including interparticle void space. It affects blast pot fill quantities, conveying system design, and production cost calculations. Bulk density varies significantly by particle shape (angular vs. spherical) and size distribution.
Bulk Density Procedure (ASTM C29)
Container Calibration
Use a calibrated cylindrical metal measure (typically 0.5 or 1.0 litre capacity). Weigh empty, clean container. Record tare mass and container volume (calibrated volume per ASTM C29 Annex).
Sample Loading — Rodded Method
Fill container in three equal layers. Rod each layer 25 times with a 16 mm diameter tamping rod using uniform strokes penetrating the previous layer. Strike off excess with a straight edge. Weigh filled container. Rodded bulk density provides the most reproducible result for specification purposes.
Calculation
Bulk Density (kg/m³) = (Mass of abrasive ÷ Container volume) × 1000. Perform test in triplicate and average results. Report to nearest 10 kg/m³. Repeat if any individual result deviates > 2% from mean.
Typical Bulk Densities
| Abrasive | Typical Bulk Density (kg/m³) | Specific Gravity |
|---|---|---|
| Steel shot / grit | 4000–4500 | 7.4–7.8 |
| Cast iron grit | 3800–4200 | 7.2–7.6 |
| Garnet (almandine) | 2200–2600 | 3.9–4.2 |
| Copper slag | 1800–2200 | 3.0–3.5 |
| Coal slag (black beauty) | 1400–1700 | 2.5–3.0 |
| Aluminum oxide | 1700–2000 | 3.8–4.0 |
| Silicon carbide | 1500–1800 | 3.1–3.2 |
| Olivine sand | 1700–2000 | 3.2–3.4 |
| Staurolite | 2200–2500 | 3.6–3.8 |
Specific Gravity (Particle Density)
Specific gravity is the ratio of the particle density to water density. Unlike bulk density, specific gravity excludes interparticle voids and characterizes the intrinsic material density. It is used for blast machine velocity calculations, production rate estimation, and identifying off-specification or adulterated material.
Pycnometer Method (ASTM C128)
A calibrated flask (pycnometer) is filled with water to a known volume. A known mass of abrasive is introduced and the volume of displaced water is measured. Specific gravity = (Dry mass of abrasive) ÷ (Mass of equal volume of water at test temperature). Temperature correction to 23°C is applied. Perform in triplicate; report mean ± range.
Moisture Content Testing
Free moisture in abrasive causes caking, clumping, blockage of blast pot metering valves, and — in metallic abrasives — surface rusting that introduces additional contamination onto blast-cleaned steel. Moisture testing is especially important for abrasives stored outdoors or in humid environments.
Gravimetric Moisture Method (ASTM D4643)
Sample Weighing
Weigh approximately 200 g of representative abrasive sample in a pre-dried, pre-weighed stainless steel or aluminum weighing dish. Record mass as W₁ (wet mass). Work quickly — do not allow sample to air-dry before oven placement.
Oven Drying
Place dish in forced-air convection oven at 105°C ± 5°C for minimum 2 hours. For large samples or high-moisture material, dry for 4 hours. Do not use temperatures above 120°C for metallic abrasives (may affect surface oxidation state).
Cooling and Reweighing
Transfer dish to desiccator (silica gel desiccant). Cool to room temperature (minimum 30 minutes). Weigh immediately upon removal from desiccator. Record as W₂ (dry mass). Moisture Content (%) = [(W₁ − W₂) ÷ W₁] × 100.
Acceptance Limits
Surface Profile Measurement — ASTM D4417
Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel
Surface profile (anchor pattern depth) is the direct product of abrasive size, hardness, and blast parameters. ASTM D4417 provides three methods of increasing accuracy and cost. Selection of method is specified in the coating specification or project quality plan.
Method A — Visual Comparison (Keane-Tator Comparator)
Procedure
Hold Keane-Tator Surface Profile Comparator (or equivalent ISO visual comparator) adjacent to the blast-cleaned surface. Compare surface texture under identical raking illumination angle. Select the comparator segment that most closely matches the surface. Report as the range of segments that bracket the surface appearance. Accuracy: ±1 comparator increment. Most useful for real-time process verification during blast cleaning.
Method B — Depth Micrometer with Replica Tape
Procedure
Press Testex Press-O-Film replica tape (coarse or X-coarse grade as appropriate) firmly onto blast-cleaned surface using thumb. Burnish tape with a plastic burnishing tool using 10–15 circular strokes with firm pressure until surface texture fully embosses into tape. Peel tape and measure compressed foam thickness with spring-loaded micrometer. Subtract nominal tape backing thickness (typically 50.8 µm / 2.0 mils). Report net profile depth. Accuracy: ±10% for profiles within correct tape grade range.
| Tape Grade | Profile Range | Color Code |
|---|---|---|
| Coarse | 20–64 µm (0.8–2.5 mil) | White/Green label |
| X-Coarse | 38–115 µm (1.5–4.5 mil) | White/Orange label |
| XX-Coarse | 64–152 µm (2.5–6.0 mil) | White/Red label |
Method C — Electronic Stylus Gauge (Highest Accuracy)
Procedure
Use a calibrated digital surface profilometer (e.g., Elcometer 224, Surtronic S-100, or equivalent) directly on the blast-cleaned surface. Set measurement parameters: evaluation length 12.5 mm, cut-off λc 2.5 mm per ISO 4288 unless otherwise specified. Take minimum 5 readings per measurement location, spaced > 50 mm apart. Report Rz (mean peak-to-valley) or Rt (maximum peak-to-valley) as specified. Method C provides traceable quantitative profile data suitable for engineering records and dispute resolution.
Number of Readings Required
| Area of Blast-Cleaned Steel | Minimum Readings | Distribution |
|---|---|---|
| Each 10 m² of surface | 5 readings | Random distributed across area |
| Each shift change | 3 readings at shift start | Verify blast parameters unchanged |
| After any blast parameter change | 5 readings minimum | Immediately following change |
| Dispute / non-conformance investigation | 10 readings minimum | Grid pattern across disputed area |
Particle Shape Assessment
Particle shape influences surface profile character (angular vs. peened), coating adhesion, and equipment wear. Angular abrasives (grit) produce sharp, jagged anchor patterns; spherical abrasives (shot) produce rounded, dimple profiles. Shape is assessed by visual microscopy or image analysis.
Angular/Grit Abrasives
Sharp edges, irregular fracture surfaces. Produces high Rmax values with sharp peak-to-valley profile. Required for adhesive coating systems (epoxy, zinc silicate). SAE J444 Grade: angular (<10% non-angular particles).
Spherical/Shot Abrasives
Round, smooth surface. Produces lower profile Rz values with peened, work-hardened surface. Preferred for shot peening fatigue improvement and for delicate substrates. SAE J444: > 85% round particles. Rejects: broken shot > 10%.
Physical Properties Quick Reference
| Property | Test Method | Units | Typical Range | Acceptance Basis |
|---|---|---|---|---|
| Mohs Hardness | Scratch test | Mohs scale (1–10) | 5.5–9.5 | Per abrasive type/spec |
| Vickers Hardness | ASTM E92 / ISO 6507 | HV | 390–2800 | SAE J444, ISO 11124 |
| Bulk Density | ASTM C29 | kg/m³ | 1400–4500 | Manufacturer CoA ±5% |
| Specific Gravity | ASTM C128 | g/cm³ (dimensionless) | 2.5–7.8 | Manufacturer CoA ±0.1 |
| Moisture Content | ASTM D4643 | % mass | < 0.2–0.5% | Metallic ≤ 0.2%; Mineral ≤ 0.5% |
| Surface Profile (Rz) | ASTM D4417 Method C | µm (or mil) | 25–115 µm | Per coating specification |
| Particle Shape | Visual/Microscopy | % angular or round | — | SAE J444 / project spec |