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Surface Roughness Measurement for Abrasively Finished Titanium SLM Parts: Ra, Sa, and Industry Specifications

Ra is the universal starting point for surface roughness specification on titanium SLM parts — but it is rarely the whole story. As-blasted SLM surfaces are isotropic and topographically complex; a single Ra value can describe surfaces that behave completely differently in fatigue, coating adhesion, and osseointegration. This guide covers the complete measurement framework: which parameters to specify, which instruments to use, how to set up the measurement protocol, and what the industry specifications actually require for each major titanium SLM application.

1. Why Ra Alone Is Insufficient for Blasted SLM Surfaces

The arithmetical mean roughness Ra is the most widely cited surface parameter in engineering drawings and industry standards. It averages the absolute height deviations from the mean surface over a defined evaluation length. Ra is useful, standardized, and measured by virtually every profilometer in production service. It is also the least informative single descriptor for a blasted titanium SLM surface, for three reasons.

First, Ra is insensitive to the distribution of heights. A surface with deep, narrow valleys and flat plateaus (characteristic of incompletely blasted staircase steps) can report the same Ra as a surface with a gentle, uniformly rounded profile (characteristic of a well-blasted surface), even though the two surfaces perform fundamentally differently in fatigue loading, coating adhesion, and biological response. Second, Ra from a 2D line trace is directionally sensitive on SLM surfaces: measured parallel to the build direction, Ra reflects the staircase periodicity; measured perpendicular, Ra reflects the melt track spacing. A single measurement in one direction gives an incomplete picture of the surface. Third, Ra cannot capture the areal features of complex SLM geometries — curved surfaces, lattice struts, and channel walls cannot be characterized by a straight stylus traverse. This article is part of the series on abrasive finishing for titanium SLM parts.

2. 2D Profile Parameters: Ra, Rz, Rsk, and What They Reveal

Ra (arithmetical mean height): The mean absolute deviation of the height profile from the mean line, evaluated over the assessment length. Defined in ISO 4287. Ra is the primary specification parameter for most engineering applications and is the starting point for any surface characterization. Its limitation is described above: it does not distinguish profile shape.

Rz (mean maximum profile height): The average peak-to-valley height across five consecutive sampling lengths. Rz is more sensitive than Ra to individual extreme features — deep satellite particle craters, sharp staircase-step peaks, or individual media impact craters that represent outlier events in the profile. For fatigue and coating adhesion applications, Rz better predicts behavior because crack nucleation and coating delamination are driven by worst-case surface features rather than average features. Rz is typically 5–8× Ra on well-blasted titanium SLM surfaces; on incompletely blasted or staircase-dominated surfaces, the Rz/Ra ratio may reach 10–15×.

Rsk (skewness): The third statistical moment of the height distribution, describing the asymmetry of the profile. A negative Rsk (skewed toward valleys) indicates a surface with flat plateaus and deep valleys — common on shot-peened surfaces and well-finished titanium. A positive Rsk (skewed toward peaks) indicates a surface with sharp peaks projecting above a flat datum — characteristic of incompletely blasted staircase surfaces or surfaces with residual satellite particles. Rsk < −1 after blasting indicates excellent satellite removal and staircase smoothing; Rsk > 0 after blasting is a flag to investigate whether blasting was adequate.

3. 3D Areal Parameters: ISO 25178 — Sa, Sz, Sdr, Smr

ISO 25178 defines the 3D (areal) surface texture parameters that extend the 2D profile parameters to area measurements. For titanium SLM surfaces, 3D parameters provide a more complete and representative characterization than single-line 2D profiles:

• Sa (arithmetical mean height, areal): The 3D equivalent of Ra, calculated over a defined measurement area rather than a line. Sa captures the full surface variability including features not sampled by a single profile trace. For medical implant surface characterization, Sa is increasingly required alongside Ra in purchasing specifications and regulatory submissions.
• Sz (maximum height, areal): The distance from the highest peak to the deepest valley over the entire measurement area. Sz identifies the worst-case surface feature and is the most relevant parameter for fatigue crack nucleation risk assessment.
• Sdr (developed interfacial area ratio): Indicates how much the true 3D surface area exceeds the nominal projected area, expressed as a fraction. A blasted titanium SLM surface with Sdr of 0.15 has 15% more true surface area than the projected flat area. Sdr is directly relevant to osseointegration (more surface area = more osteoblast contact sites) and coating adhesion (more bonding area = higher adhesion strength).
• Smr (material ratio): The fraction of the measurement area above a defined height threshold. Used in tribological applications to predict bearing area behavior. For titanium SLM contact surfaces, Smr at defined levels provides a standardized characterization of the plateau structure that determines the actual contact area under load.

4. Measurement Instruments: Contact vs. Optical Methods

Contact profilometry (stylus): A diamond-tipped stylus (2–5 μm tip radius) traverses the surface at a defined contact force (0.75–1.0 mN for titanium to avoid scratch-induced damage) and records the height profile. Contact profilometers are calibrated to ISO 12179, measure Ra, Rz, and profile parameters accurately on flat and gently curved surfaces, and are the standard for production inspection. Their limitations: single-line trace (misses areal features), cannot measure steep slopes beyond ±60–70° from horizontal, and the stylus tip radius limits lateral resolution to approximately 2× the tip radius (≈5–10 μm).

Optical coherence scanning interferometry (CSI/VSI): Non-contact, rapid, capable of measuring step heights from sub-nanometer to several millimeters, and able to characterize complete surface areas rather than single traces. CSI/VSI instruments measure Sa, Sz, Sdr, and the full ISO 25178 areal parameter set directly. They are the preferred instrument for characterizing complex SLM surfaces, particularly for medical implant qualification where areal parameters are required. Lateral resolution: 0.5–2 μm depending on objective magnification. Maximum measureable slope: ±70° from horizontal.

Confocal laser scanning microscopy: Provides lateral resolution down to ≈0.2 μm and is used in research and development for detailed characterization of SLA and blasted implant surfaces. Not commonly used in production due to throughput constraints (measurement of a 1 mm² area at full resolution takes 5–20 minutes).

5. Setting Up the Measurement Protocol

A complete surface roughness measurement protocol for titanium SLM finishing specifies: the parameters to be measured (Ra, Rz, Sa, etc.), the measurement instrument and its calibration traceability, the cut-off wavelength (λc) selection, the measurement location and number of replicates per part, and the acceptance criteria (pass/fail limits for each parameter).

Cut-off wavelength (λc) selection: ISO 4288 provides the standard guidance. For blasted titanium SLM surfaces, the appropriate λc depends on the Ra range: Ra 0.1–2 μm → λc = 0.8 mm (5 sampling lengths = 4 mm evaluation length); Ra 2–10 μm → λc = 2.5 mm (5 sampling lengths = 12.5 mm evaluation length). Using a λc too small for the surface Ra underestimates Ra by excluding relevant wavelengths; too large includes waviness from part geometry rather than surface texture. For most blasted titanium SLM surfaces in the Ra 1–6 μm range, λc = 0.8 mm is appropriate for the final finished surface and λc = 2.5 mm for as-built or Stage 1 blast surfaces.

Measurement location selection: Specify measurement locations on the part drawing or a separate measurement plan document. Include at least one location on each functional surface type (upskin, downskin, vertical wall) and at least one location near a geometrically complex feature (corner, fillet, channel edge) where blast coverage may be less uniform. For medical implant batch inspection, a minimum of three measurements per part in defined locations is typical; for aerospace components, the measurement plan is part of the first-article inspection record and is agreed with the customer.

6. Industry Specification Tables by Application

For detailed measurement guidance on implant surfaces, see our guide on abrasive finishing titanium SLM orthopedic implants. For shot-peened aerospace component measurement, see shot peening titanium SLM aerospace parts.

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