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To evaluate performance of coatings right quality of data is required. Various testing procedures are adopted to gather these data.
Here scratch testing, hardness testing and scanning electron microscopy are used. The objectives of these tests are to find critical load at which coating fails, to find hardness of specimen, to identify coating and substrate material.
With help of SEM thickness of coating is identified. A brief introduction is also given on coating deposition procedure.
Surface engineering includes investigation of engineering surfaces in relative motion and of related subjects and particles. In other word it is the study of excessive contact stresses which cause deformation of surfaces and failure of the components. Thus surface engineering help to produce components with the surfaces that can withstand these stresses.
Surface engineering can be defined as the application of the both traditional and innovative surface technologies to produce a composite material with properties unattainable in either the base or surface material individually. (Engineering Tribology, John Williams, 67, Cambridge 2005)
Coating used on sample no.26 is CrN
CrN is a tough thin film coating with high hardness, good oxidation resistance, and a low coefficient of friction for a ceramic coating making it ideal for metal and plastic forming applications. CrN is also more corrosion resistant than TiN, especially in aqueous solutions and is used as a direct substitute for chromium plating. The hardness of the CrN coating is approximately 2x that of conventional "hard" chrome plating.
CrN coatings are produced by cathodic arc deposition process which is a physical vapour deposition process.
In arc evaporation process a high current , low voltage arc is strikes on surface of cathode i.e. target, which gives rise to a small highly energetic area usually few microns wide known as 'cathodic spot'. The temperature at this spot is very high around 1500 Â°C which creates a high velocity jet of vaporised cathode material. The cathode spot is only active for a short period of time, then it self-extinguishes and re-ignites in a new area close to the previous crater. This behaviour causes the apparent motion of the arc. As the arc is basically a current carrying conductor it can be influenced by the application of anÂ electromagnetic field, which in practice is used to rapidly move the arc over the entire surface of the target, so that the total surface is eroded over time. One downside of the arc evaporation process is that if the cathode spot stays at an evaporative point for too long it can eject a large amount of macro-particles orÂ droplets. These droplets are detrimental to the performance of the coating as they are poorly adhered and can extend through the coating. Worse still if the cathode target material has a low melting point such asÂ aluminiumÂ the cathode spot can evaporate through the target resulting in either the target backing plate material being evaporated or cooling water entering the chamber. ThereforeÂ magnetic fieldsÂ as mentioned previously are used to control the motion of the arc.
Introduction to experimental techniques:
Several experimental tests like micro-hardness tests, scratch tests, scanning electron microscopy are being used to measure the mechanical properties of a coating.
Adhesion is the most important property of coating. Scratch testing is designed to evaluate the adhesive properties.
A diamond indenter is drawn across the coated surface while the loading rate is increasing with a constant rate, the indenter move with a constant velocity. The increasing applied load causes increase in contact stresses and finally lead to damage the coating and chipping of the coating from the substrate. The load in which the coating starts to fracture is called critical load (Lc) which is important to determine the coating adhesive properties.
Failure due to plastic deformation occurs in soft coating on hard substrate, whereas in hard coatings failure can be seen in terms of chipping, coating detachment, through thickness cracking
The failure of the coating can be detected simply by using SEM. And with a simple analogy of the results with the 'Atlas of Scratch Test Failure Mode' the failure mode could be determined which gives relative information about adhesive properties.
Indentation hardness is a non-destructive test used to evaluate the hardness of a material and measure the strength of the material in plastic deformation.
Hardness test gives the near surface characteristics of a material thus it is a good experimental test for evaluating the coating properties. Micro hardness gives the property of the surface of about microns of depth and nano-indentation gives information of the surface of depth nanometres.
The general principal of this test is to indenting the surface of a material and by measuring the geometry of the impression the hardness of the material would be determined. There are a number of techniques that based on the shapes of the indenter and applied load. Some famous techniques are Vickers hardness, Berkovich hardness and Knoop hardness. Of the three mentioned techniques Vickers and Berkovich are used extensively in special that the geometry of the indenter are designed so that the results can be directly compared with each other.
Scanning Electron Microscope (SEM)
The scanning electron microscope is a type of electron microscope that images the sample with producing a high energy beam of electrons. Different types of signals which are produced by SEM include secondary electrons (SE), backscattered electrons, characteristic X-ray. The interaction between the produced signals from SEM and atoms of the near surface of the sample generate images.
Secondary electrons from specimen has it's own characteristic x-ray, thus these electrons are used with help EDX analysis to mapping of material of coating or the substrate.
Hardness models can be categorised in two ways:
System models: this model separates the measured contact response into contributions from coating and substrate without detailed treatment of the deformation mechanisms in each.
Mechanistic models: this model considers the effect of a given deformation mechanism on the measured contact response.
The hardness test for our given test specimen was done on the Vickers hardness tester. The Vickers hardness test uses diamond indenter which is in the form of a right pyramid with a square base and an angle of 136Â° between the opposite faces. This indenter is forced into the material under the large range of varying load F, which is 1000, 500, 300, 200, 100, 50, 25. When the load is removed from the material, two diagonals d1 and d2 are calculated using a built in microscope and their mean 'd' is calculated, using the value of d, the sloping area of the indentation is calculated. The Vickers hardness is thus calculated by dividing the load applied with the sloping area of the indentation.
Vickers hardness =HV
F = Load in Kilogram-force (kgf)
D = Arithmetic mean if 2 diagonals d1 and d2 in millimetres
The condition of scratch test are mentioned as below :
ST-200 Scratch tester
Initial load:10 (N)
Final load:100 (N)
Table speed: 10 (mm/min)
Total length:9 (mm)
Initial adjustment for the test:
First clean the sample with solution then wipe it with soft tissue and allow the sample to dry for 3 minutes.
Then the sample is clamped firmly in the middle of the frictionless table so thus the load will be distributed homogeneously on the table and the sample should be level as possible.
The sample is placed under the diamond indenter by moving the frictionless table.
The table can be adjusted perpendicular to the scratch direction using manual translation table. Rotate the table until the sample is correctly positioned under the indenter.
Scratch test data is in terms of voltages, which is converted to applied load, end load and friction coefficient to obtain a plot of friction coefficient v/s load.
With thorough analysis of scratch in SEM failure mode and critical load at which coating fails are determined.
Scanning Electron Microscopy:
SEM used to observe cracks in indentation. Various images of cracks, indentation and scratch have been taken in SEM.
With help of EDX analysis coating material is identified.
Detectors used in SEM are
Everhart-Thornely secondary electron detector
Solid-state 4-quadrant detector for Back-scattered electrons
As specimen is metal accelerating voltage is between 15-20kV.
Results and interpretation:
Coating and substrate material:
From EDX analysis coating material is found out to be CrN.
Also substrate material is tool steel consisting of,
The indentation marks for different loads
Average data obtained from Vickers hardness test
The graph of composite hardness v/s load
The curve displayed here is not ideal hardness v/s load curve.
There is a sudden jump in hardness value at loads 100 and 200gf.
This difference may have occurred due to misreading of indentation diagonals.
Hardness of substrate:
At high load of 1000gf indenter penetrate the substrate,
Therefore approximate hardness of substrate is 900.67Hv.
Hardness of Coating:
At low load of 25gf indenter penetrates only in the coating,
Therefore approximate hardness of coating is 884Hv.
Images of indentations at various loads:
1 kgf load in SE and BSE
500 gf load in SE and BSE
300 gf load in SE and BSE
200 & 100 gf in BSE
50 & 25 gf in BSE
At higher loads coating deformation is dominated by cracking, we can see a nested structre of cracks around perimeter of indentation.
As load decreases cracks mostly occurs near the edges of indentation.
At low loads such as 50gf and 25gf it is hard to observe crack formation in the indentation due to low resolution.
From the image obtained from SEM it is clear that coating fails at approximately 2.37mm before the scratch ends.
Therefore approximate critical load Lc = 76.3N.
Following graph shows trends of the values obtained from the tests:
Images obtained from SEM are:
Figure cracks in the scratch
Figure recovery spallation in scratch
Figure longitudinal cracks in scratch
Figure failure of coating
From the SEM analysis and comparing with the 'Atlas of Scratch Test Failure Mode' failure mode in the coating due to scratch test is observed as Longitudinal cracks at the borders of the scratch track.
Depth of coating:
Figure Thickness of coating
From SEM thickness of coating is measured as 300Âµm.
With the help of EDX analysis coating material is found out as CrN
It's approximate hardness as per Vickers hardness test data is 884Hv
Substrate material is Tool Steel with approximate hardness 900.67Hv
As per scratch test data critical load at which coating fails is 76.3N
Thickness of coating is 300Âµm