Evidence-first editorial standard
This guide separates observed facts, engineering assumptions and data that still require confirmation. It does not promise a universal service life or replace the exact product data and project specification.
Understand what the number actually names
The grade percentage states the alumina content of the ceramic body by mass. It says nothing directly about grain size, porosity, the composition of the remaining fraction, the forming route or the firing schedule — and those are where wear behaviour largely lives. Two bodies quoted at the same percentage from different production lines are not interchangeable, and a supplier who can describe the microstructure is telling you more than one who only quotes purity.
Do not let a scratch scale settle the comparison
Ordinal scratch hardness is a coarse, ranked scale: several distinct engineering ceramics occupy the same position on it, so two alumina grades routinely appear identical there even when they are not. If hardness is genuinely part of your decision, ask for an indentation hardness with the load, indenter and specimen preparation stated. Expect the measured gap between neighbouring grades to be far narrower than the commercial framing implies.
Know which properties do move with purity, and by how little
A higher alumina fraction generally comes with a lower residual impurity content, marginally higher density and higher flexural strength; these trend together because they share the same cause. The practical consequence is usually not longer life against sliding abrasion but better behaviour in specific chemical environments, where the residual glassy phase — not the alumina — is what the medium attacks. That is the case where paying for the higher grade is easiest to justify.
Diagnose the failure before shopping the grade
Moving up a grade will not repair a lining that failed because the retention released, an unprotected edge was exposed to the stream, the shell deflected under load, or the impact angle was wrong for a rigid face. These account for a large share of premature failures, and none of them is a purity problem. Where the wear face itself is genuinely being consumed by fine sliding particles, the grade becomes a legitimate variable; everywhere else it is a distraction from the real defect.
Weigh thickness, geometry and availability against purity
A higher grade costs more per unit area and is frequently stocked in a narrower range of sizes and thicknesses. In a sliding zone with room to spare, a thicker element in the lower grade can outlast a thinner one in the higher grade for less money — the sacrificial depth is doing the work. Where clearance is fixed, the trade runs the other way. Check the stocked range and the lead time against your shutdown window before the specification hardens, because a grade you cannot get in time is not a specification.
Specify the grade the way you would specify a weld
Require the technical data sheet of the exact body being supplied, with each value tied to a named test method and specimen geometry, and treat a bare percentage as an incomplete specification. Record the grade, thickness, retention method and zone on the drawing so the next inspection compares like with like. Where the history is thin, install both candidate grades in adjacent areas of the same duty and set a measurable baseline — a side-by-side result from your own plant settles the question that no datasheet can.