The effect of friction on scratch adhesion testing : application to a sol-gel coating on polypropylene

The scratch test has long been used to study the adhesion of coatings. In this test an indenter is drawn across the surface of a coating under an increasing (continuous or stepwise) load. The load (normal to the surface) at which detachment of the coating occurs is termed the critical load. Usually, the magnitude of the critical load is related to the adhesion between the substrate and the coating by some theoretical model. It is well known that apart from the adhesion the critical load depends on several other parameters including the friction coefficient. In this paper a review of theoretical models applicable to scratch adhesion testing is given. Experimental data is used to compare the ability of these theoretical models to describe the effect of friction between the indenter and the coating on the critical load. We applied the scratch test to a model system consisting of a (hybrid) sol-gel coating deposited on polypropylene. The friction coefficient between indenter and coating was varied by a short plasma modification of the surface of the coating, while all other relevant parameters (i.e. interfacial adhesion, layer thickness, E-modulus of the coating, etc.) remained constant. The critical load (normal to the surface) showed a pronounced decrease of more than an order of magnitude with increasing friction coefficient. Several models are discussed and compared to the experimental data. In addition, the effect of substrate pretreatment on coating adhesion was studied. The adhesion of the sol-gel coating induced by microwave oxygen plasma modification of polypropylene is considerably better than the adhesion obtained by wet-chemical modification in chromo-sulfuric acid at room temperature. The adhesion induced by immersion in chromosulfuric acid is shown to be independent of the immersion time between 1 and 10 min

Pathogen-derived HLA-E bound epitopes reveal broad primary anchor pocket tolerability and conformationally malleable peptide binding

Through major histocompatibility complex class Ia leader sequence-derived (VL9) peptide binding and CD94/NKG2 receptor engagement, human leucocyte antigen E (HLA-E) reports cellular health to NK cells. Previous studies demonstrated a strong bias for VL9 binding by HLA-E, a preference subsequently supported by structural analyses. However, Mycobacteria tuberculosis (Mtb) infection and Rhesus cytomegalovirus-vectored SIV vaccinations revealed contexts where HLA-E and the rhesus homologue, Mamu-E, presented diverse pathogen-derived peptides to CD8+ T cells, respectively. Here we present crystal structures of HLA-E in complex with HIV and Mtb-derived peptides. We show that despite the presence of preferred primary anchor residues, HLA-E-bound peptides can adopt alternative conformations within the peptide binding groove. Furthermore, combined structural and mutagenesis analyses illustrate a greater tolerance for hydrophobic and polar residues in the primary pockets than previously appreciated. Finally, biochemical studies reveal HLA-E peptide binding and exchange characteristics with potential relevance to its alternative antigen presenting function in vivo