On the interpretation of tape friction

Friction between tape and guiding pins or recording heads is determined by the mechanical interaction of the microscopic asperities on the two surfaces in contact. The friction coefficient is given by f=τ_y/P_asp where τ_y is the shear strength at which the asperities yield irreversibly and P_asp is the normal stress on the asperity. From f and P_asp data in the literature, τ _y is derived. For unlubricated tapes τ_y is 10 MPa; for lubricated tapes 1-2 MPa is found. Tape deformation in a tensile tester, gives 30 MPa, whereas in a nanoindenter 14 MPa is found for a lubricated tape. Both values are too large to explain the friction data. Heads are worn against tapes, in spite of the low ratio of P _asp to hardness. Head wear depends on tape pigment, an aspect that is absent in the asperity description. It is proposed that, in order to reconcile the discrepancies, it is necessary to consider a particulate tape as a composite material. The scale of the relevant asperities is that of the pigment particles