Preparation Considerations

Cutting

Characteristics:

•      Thermal spray specimens are coating/substrate systems rather than single homogeneous materials, and the coating and substrate may respond very differently during sectioning
•      Many sprayed coatings contain inherent porosity, oxides, unmelted or partially melted particles, lamellar boundaries, and pre-existing microcracks that must be  preserved rather than created or smeared
•      The sprayed layer can delaminate, chip, or crack if it is unsupported or cut under unfavorable loading
•      The main risk during cutting is disturbing the true coating/substrate interface before the mounting step even begins

More Attention:

•      Blade selection based on the harder or more damage-sensitive constituent of the system, not just the substrate alone
•      Cutting direction and specimen orientation so the coating remains supported and, where possible, the cut keeps the coating under compression
•      Firm but padded clamping, especially near thin coatings, coated edges, or fragile test coupons
•      Coolant delivery, cutting severity, and removal of debris from the cutting zone
•      Whether sectioning has already introduced interfacial separation, edge chipping, or false crack networks

Avoid:

•      Delamination at the coating/substrate interface
•      Edge chipping, spallat ion, or breakout of the sprayed layer
•      Subsurface cracking or smeared damage that masks the true lamellar structure
•      Excessive heat or severe cutting that changes the apparent oxide or porosity distribution
•      A cut face that already distorts coating thickness or adhesion before later preparation steps

Mounting

Characteristics:

•      Many thermal spray coatings are porous, friable, or locally weak, so unsupported pores and splat boundaries can collapse or tear out during the first abrasive contact
•      Accurate evaluation commonly depends on preserving edge retention, coating thickness, interface integrity, and true pore morphology
•      Large differences in support between the coating, the substrate, and the mounting medium strongly influence later preparation damage
•      Hot compression mounting is often too severe for delicate or weakly bonded thermal spray coatings

More Attention:

•      Use of a low-viscosity castable epoxy that can penetrate pores, gaps, and crack- like features
•      Vacuum impregnation or repeated vacuum cycling for porous, friable, or highly discontinuous coatings
•      Orientation of the area of interest, especially coating thickness, multi-layer boundaries, and the coating/substrate interface
•      Adhesion, shrinkage behavior, and uniform support around coated edges
•      Stability of the specimen inside the mount before the grinding sequence starts

Avoid:

•      Shrinkage gaps that leave the coating unsupported
•      Poor impregnation that allows pores, oxides, or weak lamellae to break out during grinding
•      High-pressure mounting conditions that damage the internal structure of the coating
•      Specimen movement or interface opening inside the mount
•      Mounting artifacts that are later mistaken for real porosity, cracking, or de bonding

Grinding

Characteristics:

•      Grinding must remove sectioning damage while keeping the coating flat and fully supported against the substrate
•      Coating and substrate often grind at different rates, so relief can develop quickly if stock removal is too aggressive or poorly controlled
•      Porous, oxide-rich, or brittle coatings are more prone to pull-out and edge damage than to plastic smearing
•      For many thermal spray systems, diamond grinding provides better control than relying only on conventional SiC paper

More Attention:

•      Abrasive choice and cutting efficiency, especially when hard carbides, oxides, or ceramic spray layers are present
•      Sample orientation so abrasives cut through the coating into the substrate rather than tending to lift the coating
•      Flatness across the full cross-section, including the interface and near-edge regions
•      Thorough cleaning between steps, because porous coatings can retain loose abrasive and contamination
•      Whether each stage has fully removed prior damage before moving to a finer step

Avoid:

•      Relief between coating and substrate, or between phases within the coating
•      Pull-out of splats, carbides, oxides, or weakly bonded regions
•      Embedded debris and cross-contamination in open porosity
•      Microcracks or delamination carried forward into polishing
•      A ground surface that looks uniform but no longer represents the true coating morphology

Polishing

Characteristics:

•      Final polishing must reveal thickness, porosity, oxide distribution, splat morphology, cracks, and adhesion features without rounding or over-opening the structure
•      Because thermal spray coatings are often multiphase and heterogeneous, different constituents may polish at different rates
•      Weak lamellae, pores, and unmelted particles can be pulled out if the cloth and abrasive severity are not well matched
•      The final objective is structural fidelity of the coating cross-section, not simply a bright-looking surface

More Attention:

•      The cloth and abrasive combination for the specific coating type, especially the difference between ceramic and metallic thermal spray systems
•      Force, lubrication, and polishing direction so the coating remains supported by the substrate
•      The condition of the edge and the coating/substrate interface after fine polishing
•      Whether pores are being enlarged artificially or weak constituents are breaking away during the finishing step
•      Whether the final surface remains flat enough for reliable porosity, thickness, and adhesion assessment

Avoid:

•      Pull-out during final polishing
•      Excessive relief between hard and soft constituents
•      Artificially opened pores, dragged metal, or rounded lamellar boundaries
•      Edge rounding that changes the apparent coating thickness
•      A polished surface that appears clean but misrepresents the true microstructure