Non-Destructive Evaluation for Engineered Surfaces: Methods, In-Process Sensing, and Data-Driven Inspection in the NDE 4.0 Era

Engineered surfaces such as coatings, diffusion layers, cold-sprayed deposits, laser-clad overlays, additively manufactured surface regions, peened layers, and anodised films increasingly determine the performance, inspect ability, and service life of high-value metallic and ceramic components. Wear, corrosion, fatigue, oxidation, debonding, and crack initiation are controlled in these shallow, graded, anisotropic, and interface- dominated regions. This review examines how non-destructive evaluation (NDE) methods can be selected and qualified for them. It links inspection physics to specific problems: coating thickness and adhesion, near- surface residual stress, porosity gradients, lack of fusion, thermal-barrier-coating debonding, weak cold-spray interfaces, and additive-manufacturing surface roughness. Magnetic Barkhausen noise, eddy-current testing, active infrared thermography, phased-array and laser ultrasonics, acoustic emission, optical metrology, digital image correlation, microwave and terahertz sensing, and X-ray computed tomography are assessed for both capability and limitation. The review argues for hierarchical, multi-modal strategies in which rapid screening is followed by calibrated confirmation where required. It also covers the shift toward NDE 4.0, including in- process sensing, data fusion, physics-informed machine learning, uncertainty-aware interpretation, and digital twins. Persistent barriers include limited probability-of-detection evidence, poor transferability of data-driven models, scarce reference specimens, and incomplete acceptance criteria