Importance Of Nano Scratch Testing For Quality Control

                                                                            By: Susana Cabello
 

A major concern for coating manufacturers is their product’s ability to withstand cracking.  Once paint begins to crack, it fails to protect the substrate that it was applied on; therefore, failing to satisfy their clients.  For example, if a branch happens to scratch the side of your new truck and immediately afterwards the paint begins to chip, the coating manufacturers would lose the automotive manufacturers’ business due to their poor paint quality.  The quality of the paint is very important because if the metal under the paint is exposed it may begin to rust or corrode.  And although the paint may be resistant to cracking when first applied to metal, its properties may change over time with weathering.  This is why it’s very important to have the paint samples tested at a variety of weathered stages. Although cracking under a high load of stress may be inevitable, coatings manufacturers must be able to predict how weakening the changes may be over time and how deep the affecting scratches must be in order to provide their consumers with the best possible products. 

Measurement Objective

We must simulate the process of scratching in a controlled and monitored manner to observe sample behavior effects. In this application, the Nanovea Mechanical Tester in its nano scratch testing mode is used to measure the load required to cause failure to an approximately seven-year-old 30-50µm thick paint sample on a metal substrate.  A 20µm diamond tipped stylus is used at a progressive load ranging from 0.015 mN to 20.00 mN to scratch the coating. We performed a pre- and post-scan of the paint with 0.2 mN load in order to determine the value for the true depth of the scratch.  The true depth analyzes the plastic and elastic deformation of the sample during testing; whereas, the post-scan only analyzes the plastic deformation of the scratch.  The point where the coating fails by cracking is taken as the point of failure. We used the ASTM-D7187 as a guide to determine our testing parameters. We can conclude that having used a weathered sample; therefore, testing a paint sample at a weaker stage presented us with lower points of failure. Five tests were performed on this sample in order to determine the exact failure critical loads. (See accompanying graph.)

Scratch Tests: Protecting Truck Doors One Scratch At A Time

The scratch testing method is a very reproducible quantitative technique in which the critical loads at which failures appear are used to compare the cohesive or adhesive properties of coatings or bulk materials.  During the test, scratches are made on the sample with a sphero-conical stylus (tip radius ranging from 1 to 20µm) which is drawn at a constant speed across the sample, under a constant load, or, more commonly, a progressive load with a fixed loading rate. Sphero-conical styli are available with different radii (which describe the “sharpness” of the stylus). Common radii are from 20 to 200µm for micro/macro scratch tests, and 1 to 20µm for nano scratch tests.


When performing a progressive load test, the critical load is defined as the smallest load at which a recognizable failure occurs. In the case of a constant load test, the critical load corresponds to the load at which a regular occurrence of such failure along the track is observed.



The test specific parameters include:  
•    Loading rate
•    Scratching speed
•    Indenter tip radius
•    Indenter material   

The test specific parameters include:   
 •    Friction coefficient between surface and indenter
 •    Internal stresses in the material
      For bulk materials
 •    Material hardness and roughness
      For coating-substrate systems
 •    Substrate hardness and roughness
 •    Coating hardness and roughness
 •    Coating thickness


Surface damage can be detected through several means including microscopic observation, tangential (frictional) force recording, acoustic emission (AE) detection using wave frequencies, and depth sensing using 3-D imaging.
  
Preventing Door Scratches And Premature Coatings Failures

Nanovea Mechanical Tester, during Nano Scratch Tester Mode, allows simulation of many real-life failures of paint coatings. By applying loads in a controlled and closely monitored fashion, the instrument identifies at what specific load failure the scratch occurs. This information can then be used as a method to determine quantitative values for scratch resistance and true depth values among various samples.  A clear and consistent cracking failure that also exhibits consistent true depth values may then be used for further improvement of sample durability. The very small standard deviations also show the reproducibility of the technique of the instrument. This type of information can help manufacturers improve the quality of their coatings. The resilience of the paint on your rig door may just be improved at the nano level, thanks to nano scratch testing technology.