CERAC Coating Materials News

Volume 8, Issue 4

December, 1998

Example: Study of an Optical Coating with Electrical and Mechanical Requirements

A very interesting study was published that illustrates many of the aspects of thin film coatings having requirements for optical, electrical, and mechanical performance [5]. The subject is coating polycarbonate and glass surfaces to provide scratch resistance, electrical conductivity, and simultaneously high optical transmission. We have previously discussed how abrasion resistance is influenced by film hardness and its cohesion and substrate adhesion strengths. Film microhardness and cohesion are functions of packing density; higher density gives greater hardness and cohesion. Adhesion is determined by bonding energy and is influenced by the surface condition of the substrate (including contamination, chemical and physical bonding, etc). All of these parameters are affected by the stresses in the film and at the interface. In developing an AR coating for polycarbonate as well as glass surfaces, the authors of the paper were faced with the challenges of achieving deposition on a heat-sensitive surface. They then evaluated the mechanical properties of their coating along with the optical performance.

The coating consisted of 1000 Å ITO overcoated with various layer thicknesses of silica. These coatings were rf-magnetron sputter deposited from CERAC hot pressed oxide ceramic ITO (90 wt% In2O3 + 10 wt% SnO2) and melted silicon targets [6]. The outer silica layer thickness was varied from 1300 Å to 30,000 Å to study hardness, abrasion, and adhesion properties. These silica layers are greater than the QW thickness required for a simple two-layer AR coating, but the thrust of the reported investigation was improvement in mechanical behavior.

The sputtered silica had an amorphous microstructure while the ITO was columnar. Therefore the silica layer is expected to be more wear resistant because of its greater packing density and lower coefficient of sliding friction. What remains to be explored are the adhesion and hardness properties as a function of thickness. Because the magnitude and sign of the total (internal and interlayer) stress is affected by thickness (as well as deposition parameters), it is important to perform several diagnostic tests to distinguish adhesion, cohesion, and hardness strengths.

The researchers tested adhesion strength with a pull tester and adhesion and cohesion with a stylus scratch tester. Hardness was evaluated by the Knoop microhardness indenter technique. In the scratch test, the load on the stylus is increased as it is drawn over the coating, and an acoustic signal indicates vibration magnitude as the film is abraded and finally breaks and is detached from the substrate. Detachment indicates the “critical load”, which is typically used to quantify adhesion strength. However, the results of the scratch test for hardness and adhesion must be considered as indicative only. Microscopic examination of the scratch path assists in qualitatively determining from the nature of the film fracture the relative adhesive and cohesive strengths. Low cohesive strength is evidenced by cracking of the film by the stylus; low adhesive strength is evidenced by removal from the substrate. The onset of tensile-type cracks in a broken ring pattern precedes film spalling for hard brittle coatings on hard substrates. Brittle films crack and can be removed from the substrate by the stylus, while ductile films are deformed and thinned along the path. The interpretation is further complicated in the case of the polymer substrate because plastic deformation occurs, as is well illustrated in Ref. 5. Since the stylus sinks deeper into the substrate, compressive strain forces are created and the nature of the wear track changes dramatically between the glass and polycarbonate substrate. The coating applied to glass shows brittle fracture, while the same coating on polycarbonate exhibits deformation cracks with less removal from the substrate. One observation made during this study is that the coating with higher ductility will exhibit greater wear resistance. In spite of the different natures of the scratch damage, authors Wu and Chiou conclude that increasing the silica layer thickness increases the Knoop microhardness, and adhesion strength for both substrates.

Dr. Mitchell C. Colton, Editor
CERAC, inc.
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
e-mail: marketing@cerac.com

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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