section{
  display: flex;
  margin:2rem;
  justify-content: center;
  align-items: center;
}
.btn a {
  display: inline-block;
  text-decoration: none;
  border: 2px solid #333;
  color: #333;
  padding: .7rem 1.5rem;
  transition: .2s;
}

.btn.btn__18 a {
  overflow: hidden;
  position: relative;
  transition: .3s;
}

.btn.btn__18 a:before, .btn.btn__18 a:after {
  content: '';
  position: absolute;
  margin: auto;
  width: 100px;
  height: 100px;
  background-color: #2382e5;
  border-radius: 50%;
  z-index: -1;
  transition: .3s;
  display: block;
}

.btn.btn__18 a:before {
  top: 0;
  left: 0;
  transform: scale(0) translate(-50%, -50%);
  transform-origin: top left;
}

.btn.btn__18 a:after {
  right: 0;
  bottom: 0;
  transform: scale(0) translate(50%, 50%);
  transform-origin: bottom right;
}

.btn.btn__18 a:hover {
  color: white;
}

.btn.btn__18 a:hover:after, .btn.btn__18 a:hover:before {
  transition: .6s;
}

.btn.btn__18 a:hover:after {
  transform: scale(10) translate(50%, 50%);
}

.btn.btn__18 a:hover:before {
  transform: scale(10) translate(-50%, -50%);
}