J. Phys. Chem. B 2000, 104, 7085-7095
7085
Thermodynamics of Confined Perfluoropolyether Films on Amorphous Carbon Surfaces Determined from the Time-Dependent Evaporation Kinetics G. W. Tyndall* IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120
R. J. Waltman IBM Storage Systems DiVision, 5600 Cottle Road, San Jose, California 95193 ReceiVed: June 28, 1999; In Final Form: May 8, 2000
The thermodynamics of ultrathin (e10 Å) perfluoropolyether (PFPE) films in contact with amorphous carbon surfaces (CHx and CNx) are derived from the time-dependent kinetics of film evaporation. Two nonfunctionalized PFPE structures were studied: a polydisperse (Mw/Mn ) 1.4) Fomblin Z with an average molecular weight of 4000 g mol-1 and a fractionated (Mw/Mn ) 1.05) Demnum sample of average molecular weight 2200 g mol-1. Data is also presented for the evaporation of a fractionated (Mw/Mn ) 1.08) sample of the hydroxyl-terminated Fomblin Zdol (Mw ) 2100 g mol-1). Evaporation of the nonfunctionalized PFPEs from amorphous carbon follows nonclassical, first-order desorption kinetics having a rate constant that varies inversely with time. Evaporation of the functionalized Fomblin Zdol is also nonclassical; however, the time dependence of the rate constant deviates substantially from that characteristic of the nonfunctionalized PFPEs. These evaporation kinetics result from the increase in the surface free energy that accompanies thinning of the PFPE film. An analytic expression for the dependence of the surface free energy on film thickness and temperature is derived from the time-dependent evaporation rate. In the Fomblin Z03 + CHx system, reasonable agreement is found between the functional form of the thickness-dependent surface free energy change determined from the evaporation kinetics and that obtained from previous contact angle measurements. The temperature dependence of the free energy is used to derive expressions for the entropy and the attractive potential energy of the confined liquid film. In the case of an ultrathin, completely wetting fluid, the magnitude of the attractive potential energy increases, and the film entropy decreases, with decreasing film thickness.
Introduction Ultrathin liquid films in contact with solid surfaces are of both theoretical and technological interest. The study of molecularly-thin liquids on surfaces is intriguing since these films are examples of two-dimensional matter that can exhibit physical and dynamic properties that fundamentally differ from their bulk counterparts. The surface forces apparatus (SFA) has been used extensively to study the confinement of fluid films by solid surfaces.1-8 The results of these studies have demonstrated that increasing the level of confinement imposed on a liquid by a surface (or between two surfaces) can (a) slow the relative motion within the interfacial film,1-3 (b) induce the formation of more highly ordered “liquid” structures,4,5 and (c) result in film properties that become increasingly solidlike.6-8 When the degree of confinement becomes sufficiently large, liquidlike to solidlike phase transitions become ubiquitous. Molecularly-thin organic films are of critical importance in a number of technological applications. Such liquid films are routinely utilized at solid-solid interfaces to reduce the friction between the two surfaces. In the magnetic recording industry, amorphous carbon overcoats are used to protect the relative soft magnetic film of computer hard disks from mechanical wear induced via contacts with the magnetic recording head. The carbon overcoated surfaces of these computer hard disks are universally lubricated by a single monolayer of a “liquid” perfluoropolyether (PFPE). The presence of this molecularly-
thin PFPE film at the magnetic recording head-magnetic recording disk interface is critical to the tribological performance, and hence mechanical reliability, of the hard-disk drive.9 Previous SFA studies have demonstrated that the frictional properties of molecularly-thin films are strongly dependent on the degree of film confinement. By analogy, one might expect the tribological performance of the head-disk interface in harddisk drives to be impacted by the extent to which the molecularly-thin PFPE lubricant film is confined. Information concerning confinement in the PFPE/carbon system, and quantification of the impact of this confinement on the resulting tribology of the hard-disk drive, are therefore required for the efficient design of future lubricant systems. A number of PFPE structures are commercially available under the trade names Krytox (or Fomblin Y), Demnum, and Fomblin Z.10 The most commonly employed hard-disk lubricants are derivatives of the Fomblin Z structure, i.e., the random copolymer of perfluoromethylene oxide and perfluoroethylene oxide repeat units. Fomblin Z (XCF2O[CF2O]p[CF2CF2O]qCF2X). The Fomblin Z backbone can be terminated with a number of different end groups. The polymer terminated with X ) CF3 is referred to as the nonfunctionalized polymer and denoted simply as Fomblin Z. A variety of functionalized Fomblin Z structures are commercially available including: X ) CH2OH (Zdol), X ) COOH (Zdiac), and X ) piperonyl (AM 2001/3001). In
10.1021/jp9921957 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
7086 J. Phys. Chem. B, Vol. 104, No. 30, 2000 addition to the dispersive interactions that characterizes the adhesion between the PFPE backbone and the carbon overcoat of magnetic recording disks, the functionalized Fomblins are capable of interacting with the surface via polar interactions of the end groups (the latter interactions when formed lead to what is referred to as bonded lubricant).11-13 The additional adhesion provided by these functionalized PFPE’s minimizes lubricant loss from the magnetic recording disk thereby enhancing the integrity of the head-disk interface. Consequently, functionalized PFPEs are exclusively employed in the magnetic recording industry for hard-disk lubrication. Information on the confinement of molecularly-thin PFPE films can be derived from kinetic measurements. The bonding of molecularly-thin Zdol films to CHx, and CNx (also more recently, metal oxide surfaces) have been reported to follow nonclassical kinetics with rate “constants” that are timedependent.11-17 In all cases, the falloff in the Zdol bonding rate constants could be described via k(t) ) k0(t/t0)-h, where k0 is the initial rate constant at the initial time t0. The explicit form of the time-dependent bonding rate of molecularly-thin Zdol films, the value of the exponent h, was found to be dependent upon the nature of the confining surface. For relatively flexible Zdol backbone structures, rate constants which scaled as t-1.0 on CHx,11-13 t-0.5 on CNx,13 and t-0.25 on metal oxide surfaces were observed.14 The bonding of confined Zdol films was also sensitive to the relative flexibility of the Zdol backbone. For 10 Å Zdol films on CHx surfaces, increasing the “stiffness” of the backbone structure resulted in a transition from t-1.0 bonding kinetics characteristic of the relatively flexible PFPE backbones to t-0.5 kinetics. These results were interpreted to reflect whether the bonding occurred from a “solidlike” state of the confined Zdol film where the reaction rate was diffusion-limited in one dimension (k(t) ) kBt-0.5),15 or whether the reaction proceeded via the “liquidlike” state of the adsorbed Zdol film (k(t) ) kBt-1.0). The kinetics of PFPE evaporation from carbon surfaces can also be used to probe the confinement in these systems. The evaporation kinetics of polydisperse Zdol 2000 from a CHx surface has been reported.11 These experiments demonstrated that in the monolayer thickness regime, the Zdol evaporation kinetics could also be described via a kinetic approach employing a time-dependent rate constant of the form k(t) ) k0(t/t0)-1.0. The origin of the inverse time-dependent falloff in the Zdol 2000 evaporation rate constant was not, however, determined. In the current work, we expand on these previous PFPE evaporation studies by measuring the evaporation of molecularly-thin films (initial film thickness ) 10 Å) of both functionalized and nonfunctionalized PFPEs from two distinct amorphous carbon surfaces. In particular, we present kinetic data on the evaporation of (a) the polydisperse, nonfunctionalized, Fomblin Z03 (Mw ) 4000 g mol-1) from both CHx and CNx surfaces, (b) a fractionated sample of the nonfunctionalized Demnum (Mw ) 2200 g mol-1) structure from CHx, and (c) a fractionated sample of the hydroxyl-terminated Fomblin Zdol (Mw ) 2100 g mol-1) from CHx. In all cases, the observed temporal evolution of the evaporation profiles indicate a reduction in the evaporation rate, and hence increase in the activation energy, with decreasing film thickness. From the time-dependent kinetic equations developed to describe the experimental results, analytic expressions for the film thickness dependent activation energies for PFPE evaporation are determined. The functional form of the increase in the activation energy with decreasing PFPE film thickness is found to be reflective of the change in the surface free energy that accompanies the thinning of confined PFPE
Tyndall and Waltman films. The dependence of the PFPE film free energy on interface temperature is then used to derive analytic expressions for the entropy, potential energy, and heat capacity of PFPE films confined by carbon surfaces. Experimental Section The carbon substrates used in these studies were 95 mm diameter computer hard disks obtained from IBM disk manufacturing. These disks are made from an Al-Mg substrate (rms roughness of ∼15 Å) onto which are sputter-deposited an underlayer of Cr, a cobalt-based magnetic layer, and nominally 100 Å of amorphous, hydrogenated carbon (CHx), or amorphous, nitrogenated carbon (CNx). The hydrogen content in the CHx films was determined via Rutherford backscattering to be 35 at. %. XPS analysis of the CHx reveals a surface that is partially oxidized (10-15% oxygen). The CNx film surface is comprised of 11 at. % nitrogen and 4 at. % oxygen (XPS). RBS was used to verify that within the detection limits of the apparatus (
