What Is Hard Surfacing and HVOF Coating on a Centrifuge?

Hard surfacing, HVOF thermal spray coatings and tile replacement are three different ways to rebuild a centrifuge’s abrasion-worn surfaces. Sentrimax applies each as a targeted repair during an overhaul, chosen by the wear pattern, the base material and how the surface needs to perform, not as interchangeable options for the same problem.

Why Wear Surfaces Fail Before the Rest of the Machine

The surfaces of a rotating assembly that contact abrasive feed – the interior of the bowl, the conveyor flights, feed ports and other wetted areas – wear at a much faster rate than bearings, shafts or housings. That wear is usually gradual and localized, which means the underlying part is often still structurally sound long after its wear surface has thinned. Rebuilding that surface, rather than replacing the whole component, is frequently the more sensible repair. Sentrimax offers three distinct ways to do it, and they are not interchangeable.

The conveyor flights typically show the most concentrated wear, since they are in constant contact with solids being moved along the bowl toward the discharge ports, and the leading edges take the brunt of it. Feed ports and discharge zones wear differently again, often showing erosion patterns from the velocity and angle at which solids pass through rather than the steady sliding contact seen on the flights. Recognizing which pattern is present is part of what determines which rebuild method is appropriate, which is why this decision is made at inspection rather than from a general description of “wear.”

Surface Prep and Inspection Before Any Coating or Weld Work

None of the three methods below are applied to a surface that has not first been properly assessed. Sentrimax’s Stage 1 inspection process includes steam cleaning and polishing of critical surfaces specifically so that wear can be evaluated accurately, along with measurement of critical precision fits. Only once the base material, remaining thickness and wear pattern are known does it make sense to choose between hard surfacing, HVOF coating and tile replacement – applying any of them without that groundwork risks building a new wear layer on top of a surface that has already lost too much material, or choosing a method that does not match how the part is actually wearing.

Hard Surfacing (Weld Overlay)

Hard surfacing applies a wear-resistant weld overlay directly onto the base metal, using MIG, TIG or laser welding depending on the geometry and the alloy involved. It builds up a new wear layer that is metallurgically bonded to the part, which makes it well suited to surfaces that see both abrasion and some mechanical impact, and to repairs on components with complex or curved geometry where a sprayed coating would be harder to apply evenly. Because the overlay is welded on, it can be built up in multiple passes to restore significant material loss on a heavily worn flight or edge, and it tends to tolerate the odd impact from tramp material in the feed better than a sprayed coating would. The trade-off is that the heat of welding affects the base metal in and around the weld, which is why hard surfacing is generally chosen for robust structural components rather than thin-walled or highly precise parts.

HVOF Thermal Spray Coatings

HVOF (high-velocity oxy-fuel) thermal spray propels a coating material at high velocity onto the prepared surface, producing a dense, hard coating with lower heat input into the base part than welding. That lower heat input matters on precision components where warping or metallurgical changes from welding would be a concern. HVOF coatings are generally applied to surfaces where a controlled, uniform coating thickness is important and the part’s dimensional precision has to be preserved. The coating bonds mechanically to the prepared surface rather than fusing into the base metal the way a weld overlay does, which makes surface preparation – cleaning and roughening the substrate correctly – a critical step in how well the finished coating holds up in service.

Tile Replacement

Some centrifuge designs use replaceable wear tiles rather than a weldable or coatable substrate on their most heavily abraded surfaces. Where that is the original design approach, Sentrimax fits new tiles as part of an OEM-spec parts package during the overhaul, rather than trying to weld or spray-coat a component that was engineered around a replaceable wear part in the first place. Tile systems are typically found on high-wear zones subjected to the most direct abrasive contact, and replacing them restores the manufacturer’s original wear-protection design rather than substituting a different repair approach onto a part that was not built to be welded or coated.

How the Right Method Is Chosen

Four factors typically drive the decision at inspection: how much of the original wear pattern is impact versus pure abrasion, how much dimensional precision the finished surface needs to hold, whether the component’s geometry suits a welded build-up or a sprayed coating more naturally, and whether the part was originally designed around a replaceable tile system in the first place. None of these can be judged from a description of the symptom alone – a machine “wearing quickly” could point to any of the three methods depending on where and how the wear is occurring.

Choosing between them is not a matter of preference. It depends on how the original component was designed, what the inspection finds in terms of wear pattern and remaining base material, and how the part needs to perform once it goes back into service. This is a decision made during the Stage 1 inspection of an overhaul, not before the part has been measured.

Machining and Balancing After Surface Work

Both hard surfacing and HVOF coating add material to a surface, which means the part is oversized relative to its finished dimension once the weld overlay or coating has been applied. Sentrimax’s machine shop mills, turns or grinds the built-up surface back down to the correct OEM dimension and fit, rather than leaving it to be worn back into shape by the process itself. Because these repairs are performed on rotating components, the assembly is checked and, where needed, rebalanced to ISO G 0.5 after machining, using the same three-plane balancing standard applied to every rotating assembly Sentrimax rebuilds.

How This Fits Into an Overhaul

Wear surface rebuilding is typically one line item within a broader overhaul scope rather than a standalone service. What is included in a full centrifuge overhaul covers where surface rebuilding sits alongside machining, balancing and the rest of the restoration.

When Wear Protection Alone Isn’t Enough

Rebuilding a wear surface addresses abrasion damage, but it does not correct imbalance, bearing wear, or fits that have opened up beyond OEM tolerance elsewhere in the assembly. If oil analysis or vibration trends are already pointing to broader wear, a surface rebuild on its own is unlikely to be a complete fix. How often a decanter centrifuge should be overhauled covers how to read those signals and decide whether the job in front of you is a surface repair or a full overhaul. The centrifuge overhaul and rebuild hub has the full picture of how Sentrimax scopes and sequences that work.

Tracking Wear Surfaces Between Overhauls

Wear surface condition does not have to be a surprise found only when a machine comes in for a major overhaul. Visual and dimensional checks of the most exposed surfaces can be built into an ongoing maintenance program, alongside oil analysis and vibration monitoring, so a surface rebuild can be scheduled while hard surfacing or a coating is still a straightforward option rather than after the base material has worn through.

Signs a Wear Surface Needs Attention

• Visible thinning or grooving on conveyor flights or bowl interior surfaces
• A drop in throughput or cake dryness with no other obvious cause
• Feed ports or discharge zones showing erosion rather than uniform wear
• Previous coating or overlay work showing signs of flaking or lifting
• Oil analysis or vibration trends that have started to move, which can follow once wear surfaces have thinned enough to affect balance
• A visible change in centrate clarity, which can follow once internal geometry has been altered by wear

Getting Wear Surfaces Assessed

The right method – hard surfacing, HVOF coating or tile replacement – is determined once the part has been inspected and its base material and wear pattern are known. Sending the unit in for inspection is the starting point for that decision. Because Sentrimax services all of the major centrifuge brands rather than one manufacturer’s product line, the same three wear-protection options are available regardless of whether the unit on the shop floor is a Sharples, an Alfa Laval, a GEA or another make.