Erosion-Corrosion of Superlattice PVD CrN/NbN Coatings Yashodhan Purandare Tribology Group Mechanical Engineering Department.

Tribology Group at Strathclyde : Group headed by Prof. Margaret Stack. PhD students: 1. Mathew T. Mathew 2. Buddhadev Jana. 3. Yashodhan Purandare. 4. Owen Fozzard. (undergrad. student) Visiting Scientists: 1. Dr. Hussein Jawan. 2. Dr. Weijiu Huang.

Progressing Projects: Micro-abrasion-corrosion mapping of pure metals in aqueous slurries Micro-abrasion mapping of bio-materials Mapping micro-abrasion interactions in bio-oils Nanoscale studies of erosion-corrosion of "superlattice" PVD coatings in aqueous conditions

Projects continued An impinging jet study of the erosion-corrosion of MMC composite based coatings in aqueous conditions Predictive modelling of erosion-corrosion of metallic and composite materials in aqueous slurries Sliding wear-corrosion mapping in aqueous conditions Effects of applied load on the sliding-wear-corrosion map in aqueous conditions

Facilities: Impinging Jet Erosion Apparatus Rotating Cylinder Electrode Apparatus Micro-Abrasion Apparatus (Plint TE66) Pin on ring apparatus for sliding in corrosive conditions

Impinging Jet Erosion Apparatus

Rig Highlights: Reuse of the erodent particles. Minimum erosive loss of the system. Protection of the pump and the pipeline from the erosion. Small size samples.

….continued Use of Erodent particles from size microns. Easy control of Impact velocity of slurry and impact angle. Flexibility of Impact velocity from m/sec. Computer controlled Potentiostat for corrosion experiments. Impact angle adjustment from 10 degrees to 90 degrees.

Calibration of rig: ‘Heart’ of the rig.

Calibration curves:

Erosion of Pure Metals

Erosion of WC-Co-Cr Cermets:

Atomic force Microscopy

……………AFM

Superlattice Coatings Schematic Superlatice Coating microstructure

Superlattice coatings: Surperlattice coatings are the extrinsic super hard coatings - hardness of the coatings exceeds 40Gpa. These coatings have a high oxidation resistance and can be used upto 700ºC, e.g. TiAlN coating. The total thickness of the multilayer coatings is in the range of few microns.

…...continued The bilayer of the coating is known as the superlattice period. A typical superlattice period can be around 1.6 – 4 nm. Coatings can be divided into five groups depending on the composition of the bilayer.  Metal superlattices  Nitride superlattices  Carbide superlattices  Oxide superlattices  Nitride, Carbide, or Oxide/ Metal superlattices

CrN/NbN superlattice coatings: CrN/NbN coatings have been manufactured at the Engineering Group at Sheffield Hallam University, led by Dr Papken Hovsepian. CrN has superior mechanical properties where as NbN has excellent corrosion resistance. The combination of both produce excellent tribological properties. There has been very little work to investigate the performance in combined erosion -corrosion conditions.

Aim of the project Erosion: CrN/NbN superlattice coating  Study of coating performance at different impact angles of slurry.  Study of coating performance at different slurry impact velocities.  Study the wear mechanism of coating.  Construction of mechanism maps & Construction of wastage map.  Modelling to correlate impact angle, velocity & the erosion rate.

Erosion-Corrosion:CrN/NbN superlattice coating  Study performance of coatings for aqueous corrosion at different potentials.  Study performance of coatings for aqueous corrosion at different pHs.  Construction of erosion-corrosion regime maps  Construction of Wastage maps in synergism.

Results: Polarisation curves at varied degrees of impact angle of slurry.

Discussions: Results shows the effect of impact angle on the erosion behaviour. Pure metals and CrN/NbN coatings are to be investigated further for the erosion behaviour with respect to Impact angle of slurry as well as impact velocity. Polarisation curves for CrN/NbN superlattice coatings, to some extent, show a change in the synergistic wear data which is to be investigated further to nanoscale with the help of Atomic Force Microscopy (AFM).

Acknowledgement :  Tribology group at Strathclyde.  Surface Engineering Group at Sheffield Hallam University.  Staff of Mechanical Engineering in Strathclyde University.  Technical Staff of Lab M2 and M15 in Mechanical Engineering.  All those who have directly and indirectly helped me!!!

Questions & Suggestions: All suggestions welcomed!!! Address for correspondence: Prof. M. M. Stack Department of Mechanical Engineering, University of Strathclyde, James Weir Building, 75 Montrose street, Glasgow G1 1XJ.

Thank you !