Abstract An increasein the anti-armor threats has put forward the need for an increase in thedevelopment of ballistic armor protection systems or mechanisms. The mostinnovative idea or creation is nano-composite ceramics. Nano-composite ceramichas shown promise in development of armors which has got significant ballisticproperties. In order to boost the effectiveness of the nano-composite ceramicsin acting as an armor, significant research has taken place. After conductingDOP tests on various types of ceramic material it was concluded that theballistic properties of the ceramic material has increased with an increase inthe macro hardness and by using nano-composite ceramics with grain size greaterthan 0.
5µm. 1. Introduction Modern compact scenarios present itself with alot of situations that could be fatal for a soldier. It is necessary to equip asoldier with body armors. These body armors should be capable of protecting thesoldier from various lethal weapons like bullets, body armor piercing rounds,etc.Armoredvehicles were introduced to protect the soldiers from bullets and safely reachthe battleground.
But along with it came new technologies that made armoredvehicles look like vehicles made of cardboard sheets. Kinetic energy (KE)-penetrators or armor piercing (AP)-bullets, shaped charges, explosively formed penetrators (EFP) and fragments from mines andimprovised explosive device (IED) in combination with blast waves are some ofthe many threats faced by armored vehicles nowadays 1.Armorused for resisting armor piercing rounds are made from ceramic/composite”sandwich” and is called as hybrid armor. The idea ofapplying hard enamel coating on steel improved bullet resistance.
Lack oflightweight armor leads to loss of helicopters duringthe Vietnam War. This in turn leads to the development of more dedicated andwell-funded research and development. It was soon concluded that ceramics wasthe best choice for improved armor 2. Further research and development onceramics has shown that ceramics alone was not sufficient to withstand incomingprojectiles. To overcome this obstacle an energy absorbing layer wasintroduced. It was also found out that a combination of ceramic and compositeis lighter than a steel armor of the same size. This understanding lead to thedevelopment of hybrid armor which is a layer formed by the bonding of a ceramicand composite 2. Fig 1: Hybrid Armor modelThe effect ofadhesive was found to be negligible after conducting experiments on variouscomposites.
One possible method to estimate the ballistic limited velocity ofthe composite is by using an analytical model given by Florence. The model isbased on energy balance equations of the projectile and target. Kaufman et al.conducted penetration depth tests on four different types of ceramics using a12.7 mm projectile. The tests concluded that silicon carbide ceramics arestronger than alumina ceramics 2. Studiesconducted by various researchers reported that addition of nanoscale particlesin small percentage resulted in the improvement of the mechanical properties ofceramics. Niihara was able to increase the strength of alumina by uniformly distributingnano-particles inside the base structure of alumina 3.
Sadough Vanini etal. realized that addition of SiC (Silicon Carbide) particles changed theceramic fracture mode from inter-granular to trans-granular which in turnincreased the fracture toughness and strength of the ceramic 4. 2. Material Properties Ceramics actsas an effective material armor because of two main reasons: 1) Reduces theprojectile into fragments on impact 2) Distributes the impact load over alarger area of the backing material. Tile thickness Shockeyet al. found out that the initial resistance is due to the compressive strengthof the ceramic. The projectile will be fractured or deformed upon impact. Thishappens only when the strength of the ceramic exceeds the strength of the projectile 2.
This implies thatthe compressive strength of the ceramic plays a major role in fracturing theprojectile. Hardness From thework done by Rosenberg and Yesherun, it can be noted that the ability of aprojectile to penetrate the backing material can be severely decreased byblunting the projective. An observation by Den Reijer states that there is nobenefit in increasing the hardness of the projectile above the required value 2. Density Variousresearchers have stated that low density is beneficial for target material.
Lower density contribute to lighter weight armor. At the same time it allows inusing a thicker ceramic without a substantial weight penalty 2. Young’s, Bulk, Shear moduli Young’s,Bulk, Shear moduli are the main factors that determines the ability of an armorto defeat a projectile. The values of these factors that result in an effectivearmor has to be determined by conducting experimental trials on different typesof ceramics having different values of these factors 2.
Shear Strength Tensionand large stress gradients exist next to the contact area and area directlyunderneath the projectile core respectively. This implies that higher yieldstrength helps in resisting the failure due to the shear stresses produced nearthe impact site 2. 3.
Experimental Procedures Themethod used for evaluating the ballistic protection capability is done by usingDOP (Depth of Penetration) test. The target is a composite made of a ceramicunder study and a backing material. The result is then compared with theresidual penetration in a semi-infinite reference target.
This method ofexperimentation helps in determining the maximal ballistic protection potentialand also to compare between different results used for experimentation. Thebacking material used was an armor steel of medium tensile strength (1000 MPa)and also as a reference material for comparison. Because of higher stiffnesssteel was preferred over aluminum. ErhardtLach et al.
conducted ballistic tests using a tungsten heavy metal rod having ageometry of 4 mm in diameter and 60 mm in length. The impact velocity added upto 1660 m/s. Al2O3 ceramic tiles of 10 mm in thickness and Si3N4ceramic tiles of 20 mm in thickness were used for DOP tests 1. The parameters usedwhile performing DOP tests are schematically shown in the figure given below.The data measurement will be done at the target and the results procured willbe used to evaluate the results using the equations 1.
Fm = Fs= F =Fm * Fs Ali Asadiet al. has conducted experiments using alumina powder which has a purity of99.6% and has a grain size of 3 microns. SiC and MgO particles having a grainsize of 100 nanometers were added to improve the mechanical properties ofalumina. The process used to achieve this is explained using a flow chart 4.
Fig 2: Theprocess of preparing nano-composite tile ceramic Sixdifferent combinations of alumina powders combined with silicon carbides havingvolume percentages of 0, 2.5, 5, 7.5, 10, 15 and 500 ppm MgO powder solutewhere mixed in isopropyl alcohol in a planetary mill for a duration of 3 hoursinside a magnetic mixer at a temperature of 100°C and then kept for drying at atemperature of 130°C. Hydraulic press of 20 to 30 bars is used to produce nano-compositewith dimensions 120x120x12 mm 4. Variousfactors like relative density, hardness, strength, elasticity modulus andfracture were measured during the tests. The ballistic energy distributioncoefficient is calculated using D=0.36(HCE)/ Kw 2 Area-density of the panelsis evaluated using the given equation 4Areal-density ( = where n, di, tirepresents number of layers, density of separate layers and the number oflayers.
Andreas Krellet al. tells about another important parameter that influence the transmissionmeasurements: specimen thickness. Due to the thickness effect, only materialswith real-in-line transmission with a value close to the theoretical value canbe enlarged in thickness 5. Mustafa Beyila etal.
performed several tests on alumina/aluminum composite target. The heattreatment conditions that they investigated along with the thickness ofaluminum and alumina layers have shown that they have a major influence on theballistic behavior of the composite target 6. M Bolduc et al.have done DOP tests on Al2O3 ceramic composites. Thesintering of SiC + CNT was found out to be unsuccessful and also no sampleswere available for the tests 3.
The measured valuescorresponds to straight lines from the point of penetration to the deepestdistance in the backing material as shown in the figure below. Fig 3: DOP measurement Fig 4: DOP measurement 4. Results Erhardt Lach et al. after conducting DOP tests on Al2O3concluded that the results obtained after the performing evaluation of theequations with the values measured has shown that Fa valuesincreased with increase in hardness or with decrease in grain size 1. The best outcomewas obtained or seen for a grain size of 0.
6 µm. It was also concluded that themacro hardness decreased with decrease in grain size. This lead to a slightincrease in ballistic result. It was also observed that maximum in ballisticprotection was seen at the smallest grain size without massive amorphousinter-grain phases. The grain size of Al2O3 with grain size of 0.6µm and 9.82µmis shown below.
Fig 5: SEM micrographs of fractured Al2O3 withgrain size 0.6µm Fig 6: SEM micrographs of fractured Al2O3 withgrain size 9.82µm 5. Conclusion From the DOP tests conducted by the researchersmentioned in the paper, it can be concluded that the ballistic performance of anano-composite ceramic depends on the macro hardness and the type of the nano-compositeceramic used.
Since liquid phase sintering increases the amorphous phasesbetween the crystalline grains and thus restraints the increase of the macrohardness, it is always preferable to use ceramic material which could besintered in solid phase. It is also preferable to use nano-composite ceramicwith a grain size more than 0.5µm.
Use of nano-composite ceramic with a grainsize less than 0.5µm leads to the decrease in the hardness of the nano-compositeceramic material which in turn decreases the ballistic properties of thematerial. Also substituting the conventional nano-composite ceramic with Al2O3 – SiC – MgO nano-composite ceramic leads to thereduction of the areal-density of the samples by at least 30% 4.