Textured CrN thin coatings enhancing heat transfer in nucleate boiling processes

Abstract The present study reports on nucleate boiling tests performed with specifically textured chromium nitride (CrN) coatings exhibiting either predominant (1 1 1), (2 0 0) or (2 2 0) crystal lattice orientations. Those tests reveal promising results concerning thin film applications in the field of heat transfer enhancement during nucleate boiling. High power impulse magnetron sputtering (HiPIMS) in combination with direct current magnetron sputtering (DCMS) was applied to deposit CrN coatings of 4.5 μm thickness on average. A thin coating of pure chromium (Cr) was used as adhesive interlayer between substrate and CrN coating. The crystallographic phases and orientations of the coatings were determined by X-ray diffraction (XRD) in θ/2θ mode and texture coefficients T c (h k l) were calculated. High resolution scanning electron microscope (SEM) visualizes the shapes, sizes and orientations of the grains. Different Cr|CrN coating systems were deposited on pure copper heater samples and boiling curves were measured in nucleate boiling experiments in order to determine heat transfer coefficients (HTC) and critical heat flux (CHF) at dry out of the heater surface. Final comparison among the coatings' heat transfer properties reveals differences in CHF and HTC of a factor of 1.8, respectively 1.6, according to the applied microstructure.